Method of obtaining carbon dioxide from carbon dioxide-containing gas mixture

ABSTRACT

Disclosed are methods of obtaining carbon dioxide from a CO 2 -containing gas mixture. The methods combine the benefits of gas membrane separation with cryogenic temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/286,707 filed Dec. 15, 2009, U.S. Provisional Application No.61/357,597 filed Jun. 23, 2010, and U.S. Provisional Application No.61/358,865 filed Jun. 25, 2010, each of the disclosures of which areincorporated herein by reference.

BACKGROUND

Membranes have been proposed to separate CO₂ from other components ineffluent gas streams. The recovery of carbon dioxide from effluent gasstreams is propelled by multiple factors including the industrial carbondioxide market, enhanced oil recovery (EOR), and governmental andindustrial efforts to reduce greenhouse gas emissions reduction.

Many methods exist to remove CO₂ from other components in effluent gasstreams. When the effluent gas streams contain a high amount of CO₂, thestream may be cooled to provide a liquid CO₂ product. When the effluentgas streams contain a low amount of CO₂, various methods have been usedto increase the CO₂ content prior to cooling, such as membraneseparation or adsorption. Often when multiple methods are used,integration of the two methods to obtain more efficient energy savingshas been overlooked. For example, U.S. Pat. No. 4,639,257 disclosesrecovery of carbon dioxide from a gas mixture using membrane separationand distillation. However, each step is effectively performed inisolation, with temperature and pressure adjustments before eachmembrane and distillation process. As disclosed in the '257 patent, thegas temperature and pressure for membrane separation is approximately300 K (26.85° C.) and approximately 28 bar, respectively, whereas thatfor distillation is approximately −3° C. (270.15 K) to −40° C. (233.15K) and approximately 1 to 3 bar, respectively. The energy requirementsfor such a process make it inefficient.

According to the U.S. Department of Energy, no current technologyremoves at least 90% of the CO₂ from flue gases of existing pulverizedcoal (PC) power plants with less than a 35% increase in the cost ofelectricity. [DOE 2007]. The need remains for an economical, integratedCO₂ recovery process.

SUMMARY

There is disclosed a method of obtaining carbon dioxide from aCO₂-containing gas mixture. The method includes the following steps. ACO₂-containing gas mixture is obtained. The gas mixture is cooled. Thecooled gas mixture is allowed to flow into a gas separation membranemodule made of a polymeric material to produce a carbon dioxide-richstream and a carbon dioxide-lean stream. The polymeric material has aCO₂ solubility at 35° C. and 10 bar pressure of >0.03 [(cm³ of CO₂ atSTP)/(cm³ of polymeric material)(cmHg)] and a glass transitiontemperature of >210° C. The carbon dioxide-rich stream is compressed.The compressed carbon dioxide-rich stream is at least partiallycondensed through cooling. The cooled, compressed carbon dioxide-richstream is subjected to cryogenic phase separation to produce a CO₂ richliquid and a CO₂ lean vapor stream.

There is disclosed another method of obtaining carbon dioxide from aCO₂-containing gas mixture. The method includes the following steps. ACO₂-containing gas mixture is obtained. The gas mixture is cooled. Thecooled gas mixture is allowed to flow into a gas separation membranemodule made of a polymeric material to produce a carbon dioxide-richstream and a carbon dioxide-lean stream. The gas separation membrane hasa permeability of oxygen in Barrers of less than2000/(selectivity)^(3.5) for a gas mixture of 80 mole percent nitrogenand 20 mole percent oxygen at a temperature of 30° C. and at a pressureon one side of the membrane of 30 psia with a vacuum of less than 1 mmHg on the other side of the membrane, wherein selectivity is oxygen tonitrogen selectivity. Typically, the selectivity under these conditionsis in a range of from approximately 5 to approximately 9. The carbondioxide-rich stream is compressed. The compressed carbon dioxide-richstream is at least partially condensed through cooling. The cooled,compressed carbon dioxide-rich stream is subjected to cryogenic phaseseparation to produce a CO₂ rich liquid and a CO₂ lean vapor stream.

There is disclosed yet another method of obtaining carbon dioxide from aCO₂-containing gas mixture. The method includes the following steps. ACO₂-containing gas mixture is obtained. The gas mixture is cooled. Thecooled gas mixture is allowed to flow into a gas separation membranemodule. A sweep gas is directed to a permeate side of the membrane, thesweep gas having a low CO₂ concentration. A carbon dioxide-rich permeateis recovered from the membrane. A carbon dioxide-lean non-permeate isrecovered from the membrane.

There is disclosed yet another method of obtaining carbon dioxide from aCO₂-containing gas mixture. The method includes the following steps. ACO₂-containing gas mixture is obtained. The gas mixture is cooled in aheat exchanger. The cooled gas mixture is allowed to flow into a gasseparation membrane module to produce a carbon dioxide-rich permeate anda carbon dioxide-lean non-permeate. The carbon dioxide-lean stream isexpanded to produce a cold carbon dioxide-lean stream. The carbondioxide-rich permeate is compressed. The compressed carbon dioxide-richpermeate is partially condensed via cooling in the heat exchanger. Thepartially condensed compressed carbon dioxide-rich permeate is separatedinto a CO₂ rich liquid and a CO₂ lean vapor stream. Cold energy isprovided to the heat exchanger with one or more streams selected fromthe group consisting of the cold carbon dioxide-lean stream, the CO₂lean vapor stream, and a portion of the CO₂ rich liquid.

Any two or more of the above-disclosed methods may be combined toprovide an integrated method.

Any of the above-disclosed methods or integrations of any two or more ofthe above-disclosed integrated methods may include one or more of thefollowing aspects:

-   -   the polymeric material is selected from the group consisting of:        polyimides; fluoropolysulfones; poly(phenylene oxides);        poly(fluorocarbonates); and condensation polymers of        2,2,2-trifluoroacetophenone and either biphenyl or terphenyl        ether.    -   the polymeric material is a polyimide polymer or copolymer        having repeating units of formula (I):

wherein:

-   -   each R₂ is a moiety independently selected from the group of        consisting of formula (A), formula (B), formula (C) formula (D)        and mixtures thereof

-   -   -   each Z is a moiety independently selected from the group            consisting of formula (L), formula (M), formula (N) and/or a            mixture thereof

-   -   -   each R₁ is a moiety independently selected from the group            consisting of a molecular segment of formula (a), formula            (b), formula (c), formula (d), formula (e), formula (f),            formula (g), and mixtures thereof

-   -   -   each Z′ is a molecular segment independently selected from            the group consisting of formula (h), formula (j), formula            (k), formula (l), and mixtures thereof

-   -   -   each X, X₁, X₂, and X₃ is independently selected from the            group consisting of hydrogen and an alkyl group having 1 to            6 carbon atoms;        -   each Z″ is a moiety independently selected from the group            consisting of formula (m) and formula (p)

-   -   and        -   each X₅ is independently selected from the group consisting            of hydrogen, an alkyl group having 1 to 6 carbon atoms, and            a perfluoroalkyl group having 1 to 6 carbon atoms.    -   each R₁ is a molecular segment of formula (g) and each R₂        consists of formula (D).    -   each R₁ is a molecular segment of formula (e) and each R₂        consists of formula (D).    -   each R₁ is a molecular segment of formula (e) and each R₂        consists of formula (C).    -   R₁ consists of molecular segments of formulae (a) and (e) in a        1:1 ratio and each R₂ consists of formula (D).    -   each R₁ is a molecular segment of formula (a) and each R₂        consists of formula (C).    -   each R₁ is a molecular segment of formula (a) and each R₂        consists of formula (D).    -   each R₁ is a molecular segment of formula (e) and each R₂        consists of formula (C).    -   each R₁ is a molecular segment of formula ( ) and each R₂        consists of formula ( ).    -   R₁ consists of molecular segments of formulae (a) and (c) in a        4:1 ratio and each R₂ consists of formula (C).    -   R₁ is of formula (a); X, X₁, X₂, and X₃ are hydrogen; and R₂ is        of formula (D).    -   R₁ is of formula (r); X, X₁, and X₂ are methyl groups; R₂ is of        formula (C); and Z is of formula (L).    -   R₁ consists of molecular segments of formulae (a) and (c) in a        4:1 ratio; R₂ is of formula (C); and Z is of formula (L).    -   the polymeric material is BPDA-ppODA polymerized from        3,3′,4,4′-Biphenyltetracarboxylic dianhydride and        4,4′oxydianiline    -   the polymeric material is BTDA-ppODA polymerized from        3,3′,4,4′-Benzophenone tetracarboxylic dianhydride and        4,4′oxydianiline.    -   the polymeric material is PMDA-MDA polymerized from pyromellitic        dianhydride and methylene dianiline.    -   the polymeric material is a polyimide polymerized from        pyromellitic dianhydride and 4,4′-oxydianiline.    -   the polymeric material is 6FDA/BPDA+DAM polymerized from        hexafluorobisphenol        dianhydride/3,3′,4,4′-Biphenyltetracarboxylic dianhydride and        diamino mesitylene.    -   the polymeric material is 6FDA-mpODA polymerized from        hexafluorobisphenol dianhydride and 3,4′oxydianiline.    -   the polymeric material is 6FDA-ppODA polymerized from        hexafluorobisphenol dianhydride and 4,4′oxydianiline.    -   the polymeric material is 6FDA-PDA polymerized from        hexafluorobisphenol dianhydride and phenylene diamine.    -   the polymeric material is 6FDA-IPDA polymerized from        hexafluorobisphenol dianhydride and isophorone diamine.    -   the polymeric material is a polysulfone having repeating units        of formula (II)        [R₃]_(q)[R₄]_(s)  (II)    -   wherein:        -   q=s;        -   each R₃ is a moiety independently selected from the group            consisting of a molecular segment of formula t, formula u,            and formula v

-   -   -   and        -   R₄ consists of a molecular segment of formula (w):

-   -   the polysulfone has an R₃ of formula (t) and is polymerized from        2,6-dihydroxynaphthalene, tetramethyl bisphenol-A, and        bis(4-fluorophenyl)sulfone.    -   the polysulfone has an R₃ of formula (u) and is polymerized from        2,6-dihydroxynaphthalene, hexafluoro bisphenol, and        bis(4-fluorophenyl)sulfone.    -   the polysulfone has an R₃ of formula (v) and is polymerized from        2,6-dihydroxynaphthalene, tetramethyl hexafluorobisphenol, and        bis(4-fluorophenyl)sulfone.    -   the polymeric material is a poly(phenylene oxide) selected from        the group consisting of polyphenylene oxide (PPO) of formula        (III); NO₂-substituted PPO, and NH₂-substituted PPO

-   -   the polymeric material is a poly(fluorocarbonate) selected from        the group consisting of poly(tetrachlorohexafluorocarbonate) and        poly(tetrabromohexafluorocarbonate).    -   the polymeric material is a condensation polymer of        2,2,2-trifluoroacetophenone with biphenyl ether, terphenyl ether        or both biphenyl ether and terphenyl ether.    -   the CO₂-containing gas mixture is compressed to a pressure from        about 3 bar to about 60 bar prior to the cooling step.    -   the gas mixture is cooled to a temperature from about 5° C. to        about −60° C.    -   the gas mixture is cooled to a temperature from about −20° C. to        about −50° C.    -   at least 90% of the CO₂ in the CO₂-containing gas mixture is        recovered in the CO₂ rich liquid.    -   the method further comprises the step of expanding the carbon        dioxide lean stream to yield a pressure-reduced carbon dioxide        lean stream having a temperature of from about −30° C. to about        −60° C.    -   the CO₂-containing gas mixture is obtained from the flue gas of        a combustion process, from a natural gas stream, or from a CO₂        exhaust of an fermentative ethanol production plant.    -   the CO₂-containing gas mixture is obtained from the flue gas of        a combustion process and the combustion process is selected from        the group consisting of a steam methane reforming (SMR) process,        a blast furnace, and air-fired or oxygen-enhanced fossil fuel        combustion processes.    -   the combustion process is an oxygen-enhanced fossil fuel        combustion process operated in full oxycombustion or partial        oxycombustion mode.    -   the oxygen-enhanced fossil fuel combustion process is operated        in full oxycombustion mode, primary and secondary oxidants        thereof being pure oxygen or synthetic air comprising oxygen and        recycled flue gas.    -   the oxygen-enhanced fossil fuel combustion process is operated        in partial oxycombustion mode, a primary oxidant thereof being        air and a secondary oxidant thereof being synthetic air        comprising oxygen and recycled flue gas.    -   the combustion process is an air-fired fossil fuel combustion        process, the fossil fuel is coal, and the CO₂-containing gas        mixture comprising about 8% v/v to about 16% v/v CO₂.    -   the combustion process is an air-fired fossil fuel combustion        process, the fossil fuel is natural gas, and the CO₂-containing        gas mixture comprising about 3% v/v to about 10% v/v CO₂.    -   the CO₂-containing gas mixture comprising about 60% v/v to about        90% v/v CO₂.    -   the combustion process is a steam methane reforming (SMR)        process, and the CO₂-containing gas mixture comprises about 15%        v/v to about 90% v/v CO₂.    -   the combustion process is a blast furnace, and the        CO₂-containing gas mixture comprises about 20% v/v to about 90%        v/v CO₂.    -   the method further comprises the steps:        -   compressing the carbon dioxide-rich stream;        -   at least partially condensing the compressed carbon            dioxide-rich stream by cooling to produce a CO₂ rich liquid            and a CO₂ lean vapor stream;        -   expanding the CO₂ lean vapor stream; and        -   warming the expanded CO₂ lean vapor stream, the sweep gas            being the warmed expanded CO₂ lean vapor stream.    -   the method further comprises the steps:        -   cooling the carbon dioxide-lean non-permeate;        -   expanding the cooled carbon dioxide-lean non-permeate; and        -   warming the expanded cooled carbon dioxide-lean            non-permeate, the sweep gas being a portion of the warmed            expanded cooled carbon dioxide-lean non-permeate.    -   the method further comprises the step of expanding the carbon        dioxide lean stream to yield a pressure-reduced carbon dioxide        lean stream having a temperature of from about −30° C. to about        −60° C.    -   the method further comprises the step of warming the expanded        carbon dioxide lean stream at a heat exchanger, the warmed        expanded carbon dioxide lean stream being the sweep gas.    -   the method further comprises the steps of:        -   introducing the carbon dioxide lean stream to a combustion            chamber of a gas turbine whereat a fuel and an oxidant are            combusted;        -   directing the products of combustion from an outlet of the            gas turbine to a heat exchanger; and        -   exchanging heat between the colder fuel, oxidant, and carbon            dioxide lean stream upstream of the gas turbine with the            warmer products of combustion at the heat exchanger.    -   the method further comprises the step of venting a portion of        the cooled products of combustion to the atmosphere.    -   the method further comprises the steps of:        -   compressing the carbon dioxide-rich stream;        -   at least partially condensing the compressed carbon            dioxide-rich permeate by cooling to produce a CO₂ rich            liquid and a CO₂ lean vapor stream; and        -   cooling a portion of the cooled products of combustion with            cold energy from the CO₂-rich liquid, the cooled portion of            cooled products of combustion being the sweep gas.    -   the sweep gas has a CO₂ concentration lower than that of the gas        mixture.    -   the method further comprises the steps of:        -   compressing the carbon dioxide-rich stream;        -   at least partially condensing the compressed carbon            dioxide-rich permeate by cooling to produce a CO₂ rich            liquid and a CO₂ lean vapor stream;        -   cooling the carbon dioxide-lean non-permeate;        -   expanding the cooled carbon dioxide-lean non-permeate at a            cryogenic expander to produce solid carbon dioxide and a            CO₂-depleted gas in a phase separator;        -   warming the CO₂-depleted gas;        -   expanding the warmed CO₂-depleted gas at a cold expander;        -   warming the expanded warmed CO₂-depleted gas;        -   heating the warmed expanded warmed CO₂-depleted gas via heat            exchange with steam;        -   expanding the heated warmed expanded warmed CO₂-depleted gas            at hot expander to ambient or near-ambient pressure, a            portion of which is used as the sweep gas.    -   the method further comprises the step of cooling the carbon        dioxide-lean non-permeate at the heat exchanger before expansion        thereof, wherein the cold energy provided to the heat exchanger        is the the cold carbon dioxide-lean stream.    -   the cold energy provided to the heat exchanger is the CO₂ lean        vapor stream.    -   the CO₂ lean vapor stream is expanded before provision of its        cold energy to the heat exchanger.    -   the cold energy provided to the heat exchanger is a portion of        the CO₂ rich liquid.    -   the method further comprises the step of reducing pressures of        two portions of CO₂ rich liquid to thereby provide two portions        of cooled lower pressure CO₂ rich liquid, the cooled lower        pressure CO₂ rich liquid providing the cold energy to the heat        exchanger.    -   the method further comprises the step of compressing the        CO₂-containing gas mixture to a pressure ranging from        approximately 3 bar to approximately 60 bar prior to the cooling        step.    -   the method further comprises the steps of:        -   cooling the carbon dioxide-lean non-permeate at the            heat-exchanger;        -   expanding the cooled carbon dioxide-lean non-permeate at a            cryogenic expander to produce solid carbon dioxide and a            CO₂-depleted gas in a phase separator;        -   providing cold energy to the heat exchanger with the            CO₂-depleted gas to produce warmed CO₂-depleted gas;        -   expanding the warmed CO₂-depleted gas at a cold expander;    -   and        -   providing cold energy to the heat exchanger with the            expanded warmed CO₂-depleted gas.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is a schematic of an oxycoal combustion plant;

FIG. 2A is an exemplary flow diagram of the first embodiment of thedisclosed method;

FIG. 2B is a table reporting data from a simulation of the process ofFIG. 2A for a flue gas derived from air-fired coal combustion.

FIG. 2C is an exemplary flow diagram of a variation of the firstembodiment of the disclosed method;

FIG. 3 is a graphical representation of normalized CO₂ GPU over time;

FIG. 4 is a graphical representation of normalized CO₂/N₂ selectivityover time;

FIG. 5 is a graphical representation of normalized CO₂ GPU over time;

FIG. 6 is a graphical representation of normalized CO₂/N₂ selectivityover time;

FIG. 7 is a graphical representation of normalized CO₂ GPU over time;

FIG. 8 is a graphical representation of normalized CO₂/N₂ selectivityover time;

FIG. 9A is another exemplary flow diagram of the first embodiment of thedisclosed method;

FIG. 9B is a table reporting data from a simulation of the process ofFIG. 9A for a flue gas derived from air-fired coal combustion.

FIG. 10A is an exemplary flow diagram of the second embodiment of thedisclosed method;

FIG. 10B is an exemplary flow diagram of a variation of the secondembodiment of the disclosed method;

FIG. 11 is an exemplary flow diagram of the third embodiment of thedisclosed method; and

FIG. 12A is an exemplary flow diagram of the fourth through sixthembodiments of the disclosed method.

FIG. 12B is an exemplary flow diagram of a second variant of the fourththrough sixth embodiments of the disclosed method.

DESCRIPTION OF PREFERRED EMBODIMENTS

Disclosed is a method of obtaining carbon dioxide from a CO₂-containinggas mixture to provide purified CO₂. The method combines the benefits ofgas membrane separation with those of cryogenic phase separation, butintegrates the two to maximize efficiencies. For example, the disclosedmethod provides for the recovery of greater than approximately 90% ofthe CO₂ from the flue gas of an existing air-fired coal power plant witha less than approximately 35% increase in the plant's cost ofelectricity.

The CO₂-containing gas mixture may be obtained from the flue gas of acombustion process, from a natural gas stream, or from a CO₂ exhaust ofan fermentative ethanol production plant. Suitable combustion processesinclude but are not limited to steam methane reforming (SMR), blastfurnaces, and air-fired or oxygen-enhanced fossil fuel (includingnatural gas and coal) combustion processes such as power plants.

In the case of oxygen-enhanced fossil fuel combustion processes, thecombustion may be full oxycombustion or partial oxycombustion. In fulloxycombustion, the primary and secondary oxidants may be pure oxygen orsynthetic air comprising oxygen and recycled flue gas. In partialoxycombustion, the primary oxidant may be air and the secondary oxidantmay be synthetic air comprising oxygen and recycled flue gas. Pureoxygen means that the oxidant has a concentration typically found inconventional industrial oxygen production processes such as in cryogenicair separation units. The oxygen concentration of synthetic air mayrange from a concentration above that of oxygen in air to aconcentration less than pure oxygen.

The CO₂-containing gas mixture may comprise between approximately 3% v/vand approximately 90% v/v CO₂. Preferably, the CO₂-containing gasmixture comprises between approximately 8% v/v and approximately 85% v/vCO₂. Other components that may be contained within the CO₂-containinggas mixture include but are not limited to other combustion byproducts,such as water, methane, nitrogen, oxygen, argon, carbon monoxide, oxidesof sulfur, and oxides of nitrogen.

As one example, when the CO₂-containing gas mixture is the flue gas froman air-fired coal combustion plant, it typically will contain betweenapproximately 8% v/v and approximately 16% v/v CO₂, with a balance ofwater, nitrogen, oxygen, argon, carbon monoxide, oxides of sulfur, andoxides of nitrogen. In another example, an air-fired natural gascombustion plant will typically produce a CO₂-containing gas mixturecontaining between approximately 3% v/v and approximately 10% v/v CO₂,with a balance of water, methane, nitrogen, oxygen, argon, carbonmonoxide, oxides of sulfur, and oxides of nitrogen. In yet anotherexample, when the CO₂-containing gas mixture is the flue gas from anoxycoal combustion plant (i.e., coal combusted with pure oxygen orsynthetic air), it will contain between approximately 60% v/v toapproximately 90% v/v CO₂, with a balance of water, nitrogen, oxygen,argon, carbon monoxide, oxides of sulfur, and oxides of nitrogen. In yetanother example, when the CO₂-containing gas mixture is the flue gasfrom a steam methane reformer, it will contain between approximately 15%v/v and approximately 90% v/v CO₂, with a balance of water, methane,nitrogen, oxygen, argon, carbon monoxide, oxides of sulfur, and oxidesof nitrogen. In yet another example, a blast furnace will produce aCO₂-containing gas mixture containing between approximately 20% andapproximately 90% CO₂, with a balance of water, hydrogen, nitrogen,oxygen, argon, carbon monoxide, oxides of sulfur, and oxides ofnitrogen.

FIG. 1 is a schematic view of an oxycoal combustion plant. Airseparation unit 103 produces an oxygen stream 110 at a typical purity of95-98 mol % and a waste nitrogen stream 113. Oxygen stream 110 is splitinto two sub streams 111 and 112. A primary flue gas recycle stream 115passes through coal mills 108 where coal 114 is pulverized. Downstreamof the coal mills 108, substream 111 is mixed with the combinedpulverized coal/primary flue gas recycle stream and this mixture isintroduced in the burners of the boiler 100. Substream 112 is mixed withsecondary flue gas recycle stream 116 which provides the additionalballast to the burners to maintain temperatures within the furnace atacceptable levels. Boiler feedwater stream(s) 25 is introduced in theboiler 100 in order to produce steam stream(s) 118 which is expanded insteam turbine 128. As will be explained in further detail with referenceto FIGS. 2 and 9-12, boiler feedwater stream(s) 25 may first bepreheated in a compressor 20. Flue gas stream 119 rich in CO₂, typicallycontaining more than 70 mol % on a dry basis, goes through severaltreatments to remove some impurities. Unit 104 is NOx removal systemsuch as selective catalyst reduction. Unit 105 is a dust removal systemsuch as electrostatic precipitator and/or baghouse filters. Unit 106 isa desulfurization system to remove S0₂ and/or S0₃. Units 104 and 106 maynot be necessary depending on the CO₂ product specification.CO₂-containing gas mixture 1 is thus produced.

The CO₂-containing gas mixture may be treated to remove contaminants orimpurities that would negatively affect the disclosed process. Suitabletreatment methods include but are not limited to those disclosed in WO2009010690, WO 2009095581, and U.S. Published Patent Application Nos. US2009013717, US2009013868, and US2009013871, the treatment methods ofwhich are incorporated herein by reference in their entireties.Furthermore, the moisture content of the CO₂-containing gas mixtureshould be reduced to a low level to avoid freezing in the cold heatexchanger used in the disclosed method. Known drying materials andadsorbent-based processes include alumina, silica, or molecular sieves.Condensation may also be used to lower the moisture content of theCO₂-containing gas mixture.

The minimum contaminant and impurity levels desired in membraneseparation may differ from those desired in cryogenic phase separation.Therefore, one of ordinary skill in the art will recognize that thecontaminants and impurities may be removed from the CO₂-containing gasmixture once prior to both separations, both prior to the membraneseparation and prior to the cryogenic phase separation, or just prior tothe cryogenic phase separation.

Depending upon its source, the CO₂-containing gas mixture may requirecompression by a compressor to a pressure ranging from approximately 3bar to approximately 60 bar. Many treatment methods require compressionand therefore may provide the CO₂-containing gas mixture at anappropriate pressure. Compression may be performed by one or morecompressors. The compressor may be a centrifugal compressor, a screwcompressor, a reciprocating compressor, an axial compressor, etc., andcombinations thereof. In the fourth through sixth embodiment,compression may be provided by a modified gas turbine. One of ordinaryskill in the art will recognize that compression will not be necessaryfor CO₂-containing gas mixtures obtained at elevated pressures.

When compression is necessary, the increase in gas pressure isaccompanied by an increase in gas temperature. The temperature risedecreases compression efficiency and increases demands on thecompressor. Typically, cooling may be performed between stages ofcompression or after the final stage of compression. Cooling may beperformed by a direct or indirect heat exchanger. When using a heatexchanger, which may be integral to the compressor, the compressedCO₂-containing gas mixtures may be cooled indirectly by either a coolergas or a liquid stream. For example, the heat of compression may be usedto preheat water to be used in other processes, including as a boilerfeed. In the case of a fossil fuel-fired power plant, it is particularlyadvantageous to preheat the boiler feed water prior to introduction tothe power plant boiler. The cooling of the compressed CO₂-containing gasmixture and preheating of the boiler water improves compressionefficiency and decreases the fuel input required by the boiler for steamgeneration. Both boiler and compression efficiency are increased. Forexample, when the CO₂-containing gas mixture is compressed to 16 bar,sufficient heat is generated to pre-heat boiler feed water toapproximately 147° C. In a coal power plant, such pre-heating allowsmore steam turbine energy to be used for electricity generation.

In addition to any cooling that may be required after the optionalcompression step, the compressed CO₂-containing gas mixture may becooled in one or more heat exchangers to a temperature ranging fromapproximately 5° C. to approximately −120° C., preferably fromapproximately −20° C. to approximately −50° C. One of ordinary skill inthe art will recognize that some gas mixtures may freeze above −120° C.For example, certain mixtures of N₂ and CO₂ at 10 bar absolute willstart freezing at close to approximately −70° C. Therefore, thetemperature of the compressed CO₂-containing gas mixture should remainabove its freezing point. Some condensation may result from this coolingstep, which may be removed in a knock-out vessel. Indirect cooling maybe performed by one or more heat exchangers. The heat exchanger may be aconventional heat exchanger, such as a plate fin, shell-in-tube, spiralwound, or brazed aluminum plate heat exchanger, or it may be a fallingfilm evaporator as disclosed in EP 1008826, a heat exchanger derivedfrom an automobile radiator as disclosed in pending US Pat. App. No.2009/211733, or plate heat exchangers manufactured as disclosed in FR2,930,464, FR 2,930,465, and FR 2,930,466. The heat exchangers in thecited applications are all incorporated herein by reference in theirentireties. One type of brazed aluminum plate exchanger has multipleparallel cores allowing it to cool/heat any number of streams.

In one embodiment, the carbon dioxide-lean stream derived from anon-permeate in the membrane separation step may be used indirectly tocool the compressed CO₂-containing gas mixture. Additional indirectcooling may be provided by a CO₂-lean vapor stream derived from acryogenic phase separation step (described in further detail below), aCO₂-rich liquid derived from a cryogenic phase separation step(described in further detail below), and/or a CO₂-depleted gas derivedfrom a solid condensation step (described in further detail below).Optionally, the compressed CO₂-containing gas mixture may be directlycooled by combining it with the CO₂-lean vapor stream derived from thecryogenic phase separation step (described in further detail below).Surprisingly, and as will be explained in further detail below withrespect to the first embodiment, the combined cold energies of thecarbon dioxide-lean stream and the CO₂ lean vapor stream providesufficient cooling for the compressed CO₂-containing gas mixture and forthe carbon dioxide-rich stream from the membrane separation step,resulting in a cryogenic phase separation step that does not requireexternal refrigeration. This embodiment eliminates the high operatingcost of the cooling equipment cited as detrimental in the prior art andprovides for greater than 90% CO₂ capture from existing pulverizedcoal-fired power plants with not more than a 35% increase in the cost ofelectricity. Alternatively, an external cooling source may be utilizedto provide supplemental cooling to the heat exchanger.

One of ordinary skill in the art will recognize that, if multiplecompressions steps are performed, each resulting compressed stream maysubsequently be cooled in the same or different heat exchangers,resulting in multiple successive compression and cooling steps.Alternatively, the CO₂-containing gas stream may be subject to onecompression step with multiple cooling steps or multiple compressionsteps with one cooling step.

The cooled and compressed CO₂-containing gas mixture flows into a gasseparation membrane module to produce a permeate rich in carbon dioxide(from which carbon dioxide-rich stream is derived) and a non-permeate incarbon dioxide (from which a carbon dioxide-lean stream is derived).Depending upon a variety of factors, including the concentration of CO₂in the cooled and compressed CO₂-containing gas mixture, the gasseparation membrane module may utilize one or more gas separationmodules. If more than one gas separation module is utilized, they may bearranged in series, parallel, cascade, or recycle formation. The gasseparation membrane module may comprise flat sheet membranes, spiralwound flat sheet membranes, tubular tube membranes, hollow fibermembranes, and/or other membranes commonly used in industry or laterdeveloped.

When utilizing hollow fiber membranes, the cooled and compressedCO₂-containing gas mixture may be fed to the bore-side or shell-side ofthe membrane module in cross-flow or countercurrent flow. Bore side feedmay have the advantage of the most-ideal counter-current behavior withinthe bundle, resulting in the best possible module performance. Shellside feed is more tolerant to higher particulate levels.

The mixture is fed to the non-permeate side of the gas separationmembrane. The CO₂ is then separated from the gas mixture throughselective permeation of CO₂ through the gas separation membrane to thepermeate side thereof. One of ordinary skill in the art will recognizethat the non-permeate “side” or the permeate “side” of a membrane do notnecessarily mean a single side of a membrane. Rather, in the case ofmembranes that include a plurality of hollow fibers, the permeate “side”actually is considered to be the plurality of external surfaces of theindividual hollow fibers (for bore-fed membranes) or the plurality ofinner surfaces of the individual hollow fibers (for shell-fedmembranes).

A sweep gas having low CO₂ concentration may be fed to the permeate sideof the membrane where it acts to lower the partial pressure of CO₂permeating through the membrane from the cooled and compressedCO₂-containing gas mixture. Under certain conditions, the use of thesweep gas results in a more energy efficient method and requires lessmembrane area. The concentration of CO₂ in the sweep gas should be lessthan the concentration of CO₂ in the CO₂ containing gas mixture and mayeven be approximately 0%. Suitable sweep gases include but are notlimited to: dry air, dry nitrogen, dry oxygen, a portion of the carbondioxide-lean stream derived from the carbon dioxide-lean non-permeate, aportion of the CO₂ depleted gas obtained from the solid condensationstep (described in further detail below), and a portion of the CO₂ leanabsorption phase from the third embodiment (recognizing that anyresidual solvent may need to be removed prior to its use as a sweep).

One of ordinary skill will recognize that the gas separation membraneproduces a permeate stream richer in CO₂ than the feed gas stream and anon-permeate stream more dilute in CO₂ than the feed gas stream, butthat it does not provide a 100% separation of CO₂ from the cooled andcompressed CO₂-containing gas mixture (i.e. the feed gas stream). Thepercentage of CO₂ in each of the permeate and non-permeate streams willbe determined based on a variety of factors, including but not limitedto the concentration of CO₂ in the feed gas stream, the other componentscontained in the feed gas stream, the temperature and pressure of thefeed gas stream, the selectivity of the gas separation membrane, etc.The concentration of CO₂ in the carbon dioxide-rich stream is selectedto minimize the total process energy and/or costs. The concentration ofCO₂ in the carbon dioxide-lean stream is determined by the recoveryrequired for the process. For example, a carbon dioxide-lean streamcontaining between approximately 1% and approximately 2% CO₂ providesfor an approximately 90% recovery of CO₂ from the CO₂-containing gasmixture obtained from air fired coal. One of ordinary skill in the artwill recognize that the minimum amount of CO₂ recovery may beestablished by government mandates and that the optimal energy/costscenarios may not correspond to these government mandates.

The gas separation membrane may be comprised of any material known inthe field of gas separation that is selectively permeable to carbondioxide over nitrogen, including but not limited to glassy or rubberypolymers. Typical rubbery polymers include silicone rubbers. Typicalglass polymers are described below.

The gas separation membrane desirably has a permeability of oxygen inBarrers of less than 2000/(selectivity)^(3.5) for a gas mixture of 80mole percent nitrogen and 20 mole percent oxygen at a temperature of 30°C. and at a pressure on one side of the membrane of 30 psia with avacuum of less than 1 mm Hg on the other side of the membrane, whereinselectivity is oxygen to nitrogen selectivity. Typically, theselectivity under these conditions is in a range of from approximately 5to approximately 9.

Alternatively or in addition to the above-recited oxygen permeability,suitable polymeric materials for the gas separation membrane have a CO₂solubility at 35° C. and 10 bar pressure of >0.03[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of >210°C. Exemplary glassy polymers satisfying these CO₂ solubility and glasstransition temperature conditions include polyimides,fluoropolysulfones, poly(phenylene oxides), poly(fluorocarbonates), andcondensation polymers of 2,2,2-trifluoroacetophenone and either biphenylor terphenyl ether.

The polyimides may be a polymer or copolymer having repeating units offormula (I):

wherein:

each R₂ is a moiety independently selected from the group of consistingof formula (A), formula (B), formula (C) formula (D) and mixturesthereof,

each Z is a moiety independently selected from the group consisting offormula (L), formula (M), formula (N) and/or a mixture thereof

each R₁ is a moiety independently selected from the group consisting ofa molecular segment of formula (a), formula (b), formula (c), formula(d), formula (e), formula (f), formula (g), and mixtures thereof

each Z′ is a molecular segment independently selected from the groupconsisting of formula (h), formula (j), formula (k), formula (l), andmixtures thereof

each X, X₁, X₂, and X₃ is independently selected from the groupconsisting of hydrogen and an alkyl group having 1 to 6 carbon atoms;

each Z″ is a moiety independently selected from the group consisting offormula (m) and formula (p)

and

each X₅ is independently selected from the group consisting of hydrogen,an alkyl group having 1 to 6 carbon atoms, and a pefluoroalkyl grouphaving 1 to 6 carbon atoms.

Suitable polyimides include the polyimides synthesized by conventionaltechniques from the combinations of diamines/diisocyanates anddianhydrides shown in Table I where the listed formulae (a), (c), (e),and (g) of the diamine/diisocyanates and the formulae (C) and (D) of thedianhydrides correspond to the formulae (a), (c), (e), (g), (C), and (D)above in formulae (I).

TABLE 1 Diamines/Diisocyanates and Dianhydrides for polymerizingpolyimides O₂/N₂ O₂ permeability selectivity (Barrer) of of Diamine /Diisocyanate Dianhydride polyimide polyimide 3,4′ oxy dianiline (formulag)  

BPADA (formula (D) 0.45 7.23 Diamino phenylindane BPADA (formula (D)2.25 6.5 (formula e) m-phenylene diamine 3,3′,4,4′- 0.60 6.78 (formula(a) + Diamino diphenylsulfonetetracarb phenylindane (formula e) oxylicdianhydride in a 1:1 ratio (DSDA) (formula C) m-phenylene diamine BPADA(formula (D) 1.44 7.78 (formula (a) + Diamino phenylindane (formula e)in a 1:1 ratio 1,3-diaminobenzene-4- 3,3′,4,4′- 0.35 8.1 sulfonic acid(HSMPD) diphenylsulfonetetracarb (formula a) oxylic dianhydride (DSDA)(formula C) m-phenylene diamine BPADA (formula (D) 0.4 8.0 (formula (a)Diamino phenylindane 3,3',4,4'-Benzophenone 1.30 7.1 (formula e)tetracarboxylic dianhydride (formula C) methylphenylene-3,3',4,4'-Benzophenone 0.4 7.8 diisocyanate (TDI) (formulatetracarboxylic a) + 20% diphenylmethane dianhydride (formula C)diisocyanate (MDI) (formula c)

One particular polyimide is sold by Sabic Innovative Plastics IP B.V.Company under the trademark Ultem® (hereinafter the Ultem® polyimide) inwhich R₁ is of formula (a), X, X₁, X₂, and X₃ are hydrogen and R₂ is offormula (D). Ultem has a CO₂ solubility at 35° C. and 10 bar pressure of0.07904 [cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperatureof 215° C.

Another particular polyimide is sold under the trademark Matrimid®(hereinafter the Matrimid® polyimide) in which R₁ is of formula (e), X,X₁, and X₂ are methyl groups, R₂ is of formula (C), and Z is of formula(L). Matrimid has a CO₂ solubility at 35° C. and 10 bar pressure of0.056 [cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of315° C.

Another particular polymide is sold by Evonik Fibres GmbH under thetrademark P84® (hereinafter the P84® polyimide) in which R₁ is offormula (a) in 80% of the R₁'s and of formula (c) in 20% of the R₁'s, R₂is of formula (C), and Z is of formula (L). P84 has a CO₂ solubility at35° C. and 10 bar pressure of >0.07 [cm³(STP)/cm³(polymer)-cmHg] and aglass transition temperature of 316° C.

Another suitable polymide is BPDA-ppODA which may be sythesized from3,3′,4,4′-Biphenyltetracarboxylic dianhydride and 4,4′oxydianiline.BPDA-ppODA has a CO₂ solubility at 35° C. and 10 bar pressure of 0.036[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 270°C.

Another suitable polymide is BTDA-ppODA which may be sythesized from3,3′,4,4′-Benzophenone tetracarboxylic dianhydride and 4,4′oxydianiline.BTDA-ppODA has a CO₂ solubility at 35° C. and 10 bar pressure of 0.032[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 266°C.

Another suitable polymide is PMDA-MDA which may be sythesized frompyromellitic dianhydride and methylene dianiline. PMDA-MDA has a CO₂solubility at 35° C. and 10 bar pressure of 0.0447[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 338°C.

Another suitable polymide is Kapton which may be sythesized frompyromellitic dianhydride and 4,4′-oxydianiline. Kapton has a CO₂solubility at 35° C. and 10 bar pressure of 0.030977[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 400°C.

Another suitable polymide is 6FDA/BPDA+DAM which may be sythesized fromhexafluorobisphenol dianhydride/3,3′,4,4′-Biphenyltetracarboxylicdianhydride and diamino mesitylene.

Another suitable polymide is 6FDA-mpODA which may be sythesized fromhexafluorobisphenol dianhydride and 3,4′oxydianiline. 6FDA-mpODA has aCO₂ solubility at 35° C. and 10 bar pressure of 0.046[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 260°C.

Another suitable polymide is 6FDA-ppODA which may be sythesized fromhexafluorobisphenol dianhydride and 4,4′oxydianiline. 6FDA-ppODA has aCO₂ solubility at 35° C. and 10 bar pressure of 0.054[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 299°C.

Another suitable polymide is 6FDA-PDA which may be sythesized fromhexafluorobisphenol dianhydride and phenylene diamine. 6FDA-PDA has aCO₂ solubility at 35° C. and 10 bar pressure of 0.0521[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 304°C.

Another suitable polymide is 6FDA-IPDA which may be sythesized fromhexafluorobisphenol dianhydride and isophorone diamine. 6FDA-IPDA has aCO₂ solubility at 35° C. and 10 bar pressure of 0.0558[cm³(STP)/cm³(polymer)-cmHg] and a glass transition temperature of 310°C.

Suitable polyfluorosulfones include polymers having repeating units offormula (II).[R₃]_(q)[R₄]_(s)  (II)wherein:

q=s;

each R₃ is a moiety independently selected from the group consisting ofa molecular segment of formula t, formula u, and formula v.

and R4 consists of a molecular segment of formula (w):

A polysulfone having an R₃ of formula (t) is known as TM-NPSF and ispolymerized from 2,6-dihydroxynaphthalene, tetramethyl bisphenol-A, andbis(4-fluorophenyl)sulfone. A polysulfone having an R₃ of formula (u) isknown as HF-NPSF and is polymerized from 2,6-dihydroxynaphthalene,hexafluoro bisphenol, and bis(4-fluorophenyl)sulfone. A polysulfonehaving an R₃ of formula (v) is known as TMHF-NPSF and is polymerizedfrom 2,6-dihydroxynaphthalene, tetramethyl hexafluorobisphenol, andbis(4-fluorophenyl)sulfone. Those skilled in the art will well recognizehow TM-NPSF, HF-NPSF, and TMHF-NPSF are polymerized. Particularlysuitable syntheses are disclosed by C. Camacho-Zuniga, F. A.Ruiz-Trevino, S. Hernandez-Lopez, M. G. Zolotukhin, F. H. J. Maurer, A.González-Montiel, “Aromatic polysulfone copolymers for gas separationmembrane applications”, Journal of Membrane Science, 340 (2009) 221-226.

Suitable poly(phenylene oxides) include polyphenylene oxide (PPO) offormula (III), NO₂-substituted (nitrated) PPO, and NH₂-substituted(aminated) PPO.

Those skilled in the art will recognize that the preparation ofNO₂-substituted (nitrated) PPO is known in the art and may be performedas follows. Y. S. Bhole, P. B. Karadkar, U. K. Kharul, “Nitration andamination of polyphenylene oxide: Synthesis, gas sorption and permeationanalysis”, European Polymer Journal 43 (2007) 1450-1459. PPO isdissolved in a suitable solvent such as chloroform at ambienttemperature under a flow of N₂. A mixture of nitric acid and sulfuricacid is slowly added while maintaining the dissolved PPO at atemperature of 25° C. Those skilled in the art will recognize that thedegree of substitution may be increased by increasing the ratio ofnitric acid to sulfuric acid in the mixture. Following addition, thereaction mixture is stirred for 30 minutes. The formed nitrated PPO isthen precipitated onto stirred methanol and further purified bydissolution in chloroform and reprecipitation into methanol.

Those skilled in the art will recognize that the preparation ofNH₂-substituted (aminated) PPO is known in the art and may be performedas follows. Y. S. Bhole, et al. NO₂-substituted PPO is dissolved in asuitable solvent such as chloroform in a two-necked RB flask equippedwith a reflux condenser. A solution of 30 g of SnCl₂.2H₂O and 1 g of Nalin 72 ml HCl-glacial acetic acid mixture (2:1) is added in a drop wisemanner at 60° C. while stirring. After 15 minutes of addition of thereducing mixture, the polymer starts precipitating. To avoidprecipitation, a small quantity of methanol is added until the solutionbecomes clear. The resulting mixture is further refluxed for 3 hours andthen cooled to room temperature. The resultant polymer is precipitatedby pouring the reaction mixture onto a 2 N NaOH solution. Theprecipitated polymer is then water-washed until free of base. It is thenair dried for 2 days at room temperature under vacuum. The dried polymeris purified by dissolving in chloroform and precipitating in methanol.

Suitable poly(fluorocarbonates) include but are not limited topoly(tetrachlorohexafluorocarbonate) andpoly(tetrabromohexafluorocarbonate).

Those skilled in the art will recognize that the condensation polymersof 2,2,2-trifluoroacetophenone with biphenyl, terphenyl or both biphenyland terphenyl ethers are known in the art as TFP BPE, TFP TPE, and TFPBPE/TPE, respectively, and may be synthesized as follows. M. T.Guzmán-Gutiérrez, M. G. Zolotukhin, D. Fritsch, F. A. Ruiz-Trevino, G.Cedillo, E. Fregoso-lsrael, C. Ortiz-Estrada, J. Chavez, C. Kudla,“Synthesis and gas transport properties of new aromatic 3F polymers”,Journal of Membrane Science 323 (2008) 379-385. In a typical synthesis,equimolar amounts of trifluoroacetophenone and either biphenyl,terphenyl or a mixture of biphenyl and terphenyl are dissolved indichloromethane. Trifluoromethanesulfonic acid is added and the mixturestirred at room temperature for 72 hours. The resulting green,two-phase, system is then poured slowly into methanol. The precipitated,pale white solid is filtered off, extracted with refluxing methanol andacetone, and then dried at 100° C. under vacuum.

The above polymers (polyimides, fluoropolysulfones, poly (phenyleneoxides and condensation polymers of 2,2,2-trifluoroacetophenone andeither biphenyl or terphenyl ether) may be expected to exhibit enhancedproperties at relatively low temperatures. When a gas separationmembrane made of one of the above polymers is fed with a CO₂-containingfeed gas at temperatures less than approximately −10° C., the CO₂permeance is approximately two times higher than the value that would bepredicted by a simple Arrhenius extrapolation of super-ambient (20°-50°C.) temperature data. The CO₂/N₂ selectivity continues to increase astemperature decreases.

Unexpectedly, and as shown in the examples that follow, a gas separationmembrane utilizing certain of the above-described polymers exhibit anincrease in flux without the expected decrease in selectivity during thepassage of time. Other of the above-described polymers are expected toexhibit the same unexpected advantage. As a result, the CO₂ permeance at−40° C. is similar or even higher than the permeance at ambienttemperature. In addition, the CO₂/N₂ or CO₂/O₂ selectivity at the coldertemperature is approximately two to approximately four times higher thanat the ambient temperature value. Preferably, over time, the membranesexhibit an increase in flux with a parallel increase or slight decreasein selectivity, the decrease ranging from approximately 0% toapproximately 5%. The exact mechanism by which the CO₂ permeanceincreases without loss of selectivity remains unknown.

Gas separation membranes made of the above-described polymeric materialsshow stable or even increased CO₂ selectivity in these long-term testseven as the CO₂ permeance increases. This is a critical differencebetween the low temperature conditioning phenomena observed here and thebetter known plasticization phenomena that is often a problem in CO₂separation by membranes. At low temperatures and moderate feedpressures, CO₂ activity (˜CO₂ partial pressure/saturation pressure) ishigh (up to 0.65). Plasticization, which is typically attributed toincreased mobility of the polymer matrix caused by the high CO₂activity, results in increased CO₂ permeance, and with conventionalpolymeric materials, reduced selectivity with air gases. In contrast,the selectivity of gas separation membranes made of the above-describedpolymeric materials, and at the low temperature and high CO₂ activityoperation proposed herein, does not decrease even when CO₂ permeanceincreases markedly. The net effect of cold temperature operation is asif a new material had been discovered with unprecedentedpermeability-selectivity characteristics on the Robeson trade-off plot

After the permeation process, the carbon dioxide-rich stream iscompressed by a compressor to a pressure ranging from approximately 15bar to approximately 30 bar. Compression may be performed by one or morecompressors. The compressor may be a centrifugal compressor, a screwcompressor, a reciprocating compressor, an axial compressor, etc., andcombinations thereof. The compressed carbon dioxide-rich stream is thencooled to a temperature that yields a liquid vapor mixture. One ofordinary skill in the art will recognize that the specific temperaturethat yields the liquid vapor mixture will depend upon multiple factors,including the concentration of CO₂ in the compressed carbon dioxide-richstream, the pressure of the compressed carbon dioxide-rich stream, etc.The cooling may be performed by direct or indirect heat exchange withother fluids in the process (described in further detail below).

In one particularly preferred embodiment, the carbon dioxide-lean streamfrom the membrane separation step (before and/or after expansionthereof) may be used to cool the compressed carbon dioxide-rich streamvia a heat exchanger. Additional cooling may be provided by CO₂ leanvapor stream from a cryogenic phase separation step. Surprisingly, andas will be explained in further detail below with respect to the firstembodiment, the combined cold energies of the carbon dioxide-lean stream(before and/or after expansion thereof) and the CO₂ lean vapor streamprovide sufficient cooling for the compressed CO₂-containing gas mixtureand the compressed carbon dioxide-rich stream such that the compressedcarbon dioxide-rich stream may be subjected to cryogenic phaseseparation without requiring external refrigeration. This embodimenteliminates the high operating cost of the cooling equipment cited asdetrimental in the prior art and provides for greater than 90% CO₂capture from existing pulverized coal-fired power plants with not morethan a 35% increase in the cost of electricity. Alternatively or inaddition to cooling by either of the carbon dioxide-lean stream and theCO₂ lean vapor stream, the CO₂ rich liquid from the cryogenic phaseseparation step may also be used to provide cooling to the compressedcarbon dioxide-rich stream via the heat exchanger. If desired, inanother alternate embodiment an external cooling source may be utilizedto provide cooling to the heat exchanger.

One of ordinary skill in the art will recognize that, if multiplecompressions steps are performed, each resulting compressed stream maysubsequently be cooled, resulting in multiple successive compression andcooling steps. Alternatively, the carbon dioxide-rich stream may besubject to one compression step with multiple cooling steps or multiplecompression steps with one cooling step.

The compressed carbon dioxide-rich stream may be cooled to a temperaturelower than that of the CO₂-containing gas mixture fed to the membrane.Smaller amounts of contaminants and impurities may negatively affectprocessing of the carbon dioxide-rich stream at such temperatures. Assuch, the compressed carbon dioxide-rich stream may require a greaterreduction of contaminants or impurities than was required for theCO₂-containing gas mixture. As the compressed carbon dioxide-rich streamis smaller than the CO₂-containing gas mixture, it may be beneficial toperform two contaminant removal steps. The first contaminant removalstep treats the CO₂-containing gas mixture to remove contaminants to thelevel required for the cold membrane operation. The second contaminantremoval step subsequently treats the compressed carbon dioxide-richstream to remove contaminants to the level required for processing thecarbon dioxide-rich stream.

The compressed and cooled carbon dioxide-rich stream is then subjectedto cryogenic phase separation to produce a CO₂ rich liquid and a CO₂lean vapor stream. Cryogenic phase separation is performed at atemperature typically ranging from approximately −30° C. toapproximately −150° C., and preferably at a temperature ranging fromapproximately −45° C. to approximately −120° C. The cryogenic phaseseparation may be a liquid separator. If it is desirable to removeincondensible components in the CO₂-rich liquid (such as N₂, Ar and/orO₂ dissolved in the CO₂-rich liquid) and thereby produce higher CO₂purities, the CO₂-rich liquid may be further subjected to cryogenicdistillation performed in a distillation or fractionation column orzone. In this latter case of cryogenic distillation, through heatexchange with other fluids—such as the non-permeate (before and/or afterexpansion), the CO₂-lean vapor stream from the cryogenic phaseseparator, and/or the CO₂-rich liquid from the cryogenic phaseseparator—in the process, the compressed and cooled carbon dioxide-richstream receives all of the cooling needed for the cryogenic phaseseparation. No other cooling of the cryogenic phase separator is needed,although external heating conventional to distillation columns may beprovided. The cryogenic phase separation step divides a CO₂-lean vaporfraction from a CO₂-rich liquid fraction of the compressed and cooledcarbon dioxide-rich stream to provide the CO₂-lean vapor stream and theCO₂-rich liquid.

After separation, the CO₂-rich liquid is pumped to a pressure so that,if it is warmed to room temperature, it remains a liquid. Preferably,the CO₂-rich liquid is pumped to a pressure ranging from approximately40 bar to approximately 80 bar, more preferably to approximately 60 bar.The CO₂ rich liquid passes through the heat exchanger and is provided atapproximately 60 bar and approximately 20° C. In one embodiment, one ormore portions of the CO₂-rich liquid are pumped to one or morepressure-reduction valves where the pressure of the CO₂-rich liquid isreduced to a desired level. In the event that a distillation orfractional column or zone is utilized in conjunction with the cryogenicphase separator, the especially CO2-lean vapor from the top of thecolumn or zone may be used as yet another source of cooling for the oneor more heat exchangers. As described in further detail below, theresultant optional reduced pressure CO₂-rich liquid streams may beutilized in solid condensation of CO₂ vapor and/or liquefaction of solidCO₂.

The temperature of the CO₂ lean vapor stream remains low. Therefore, asdiscussed previously, the CO₂ lean vapor stream may be utilized as acooling source (before and/or after expansion) in one or more heatexchangers. Furthermore, the carbon dioxide content of the CO₂ leanvapor stream may be approximately 10% v/v to approximately 50% v/v andits pressure remains between approximately 15 bar to approximately 30bar. To reduce the pressure, the CO₂ lean vapor stream may be warmed,expanded, and then combined with the CO₂-containing gas mixture foradditional processing and recovery.

After the permeation process, the temperature of the carbon dioxide-leannon-permeate stream remains low and may possibly be lower than that ofthe cooled and compressed CO₂-containing gas mixture. For example, thetemperature of the carbon dioxide-lean non-permate stream may range fromapproximately 1° C. to approximately 20° C. cooler than that of thecooled and compressed CO₂-containing gas mixture. As a result, and asdiscussed previously, the carbon dioxide-lean non-permeate stream mayoptionally be used to provide a cooling source for other fluids in theprocess—such as the cooled and compressed CO₂-containing gas mixture—viaone or more heat exchangers.

The carbon dioxide-lean non-permeate stream remains at high pressureafter the permeation process and may be subjected to expansion in one ormultiple stages to cover cold energy and/or to produce the mechanicalenergy required to compress the CO₂-containing gas mixture. The singleor multiple expansions may be performed on the carbon dioxide-leannon-permeate stream by one or more pressure reduction devices. Whetheror not the carbon dioxide-lean non-permeate stream passes through theoptional heat exchanger(s), the carbon dioxide-lean non-permeate streamreaches the pressure reduction device at a temperature ranging fromabout 0° C. to about −55° C. and at a pressure ranging from about 1.5bar to about 30 bar. When utilizing multiple pressure reduction devices,the carbon dioxide-lean non-permeate stream may be subjected to astepwise reduction in pressure, producing multiple expanded carbondioxide-lean streams. The expansion may be performed by any pressurereduction device including but not limited to conventionalturbo-expanders, Joule-Thomson valves, reciprocating expansion engines,centrifugal or axial flow turbines, etc., and any combinations thereof.The pressure reduction device may be a turbo-expander used to harnessmechanical energy from the expansion and provide power to othercomponents, such as the compressor for compressing the CO₂-containinggas mixture or the CO₂-rich permeate stream. One of ordinary skill inthe art will be capable of optimizing the number of pressure reductiondevices for the desired thermodynamic outcome.

Liquid CO₂

In a first embodiment, the gas separation membrane produces a carbondioxide-rich permeate stream containing approximately 50% v/v toapproximately 95% v/v carbon dioxide. The carbon dioxide-leannon-permeate stream may then be expanded to a pressure providing atemperature suitable to provide sufficient cold energy for partialliquid condensation of carbon dioxide in the carbon dioxide-rich stream.Typically, such expanded CO₂-lean non-permeate temperatures range fromapproximately −30° C. to approximately −70° C. Preferably, the carbondioxide-lean non-permeate stream leaves the expander at a temperatureranging from about −50° C. to about −57° C. and at a pressure rangingfrom less than about 30 bar to about atmospheric pressure. Thethus-produced one or more cold carbon dioxide-lean streams flow into oneor more heat exchangers to provide indirect cooling to theCO₂-containing gas mixture and/or the compressed carbon dioxide-richstream. As in the compression and cooling of the CO₂-containing gasstream, one of ordinary skill in the art will recognize that, ifmultiple pressure reduction steps are performed, each resulting streammay subsequently provide cooling to and be warmed in one or more heatexchangers, resulting in multiple successive expansion and warmingsteps. Alternatively, the cold carbon dioxide-lean stream may be subjectto one expansion step with multiple warming steps or multiple expansionsteps with one warming step.

As discussed previously, the combination of the CO₂ lean vapor streamand the carbon dioxide-lean stream, and more particularly the expansionof the carbon dioxide-lean stream, provides sufficient cooling for thecompressed CO₂-containing gas mixture and the compressed carbondioxide-rich stream, resulting in a cryogenic phase separation step thatdoes not require external refrigeration. This embodiment eliminates thehigh operating cost of the cooling equipment cited as detrimental in theprior art and provides for greater than 90% CO₂ capture from existingpulverized coal-fired power plants with not more than a 35% increase inthe cost of electricity.

Solid CO₂

In a second embodiment, the gas separation membrane produces a carbondioxide-rich stream containing approximately 50% v/v to approximately95% v/v carbon dioxide and/or a carbon dioxide-lean stream containingapproximately 4% v/v to approximately 15% v/v carbon dioxide. As in thefirst embodiment, the carbon dioxide-rich stream is compressed andcooled to produce a liquid vapor mixture that is subsequently separatedto produce the CO₂ rich liquid. In this embodiment, the carbondioxide-lean stream may be expanded to a temperature suitable for solidcondensation (also called desublimation or anti-sublimation) of thecarbon dioxide in the carbon dioxide-lean stream.

The carbon dioxide-lean stream may be fed to a cryogenic turbo expandersimilar to those used in air separation units (i.e. a radial wheel). Thepressure of the carbon dioxide-lean stream is reduced to atmosphericpressure in a single stage, while its temperature is dropped to a rangefrom approximately −90° C. to approximately −120° C. During theexpansion, CO₂ snow will be formed, leaving behind a CO₂ depleted gascontaining approximately 0.5% v/v to approximately 5% v/v CO₂. Theoutlet of the cryogenic turbo expander includes a gas/solid separator.The CO₂ snow will be extracted from the separator by an auger screw,which may serve to elevate the pressure of the CO₂ snow to its triplepoint (5.2 bar abs). The latent heat of liquefaction may be used to coolthe flue gas or to condense the carbon dioxide-rich stream aftercompression, or a warmed, vaporized portion of CO₂-rich liquid may beused to directly heat the solid carbon dioxide to liquefy it. Thethus-liquefied carbon dioxide may be mixed with the CO₂-rich liquid.Alternatively, the CO₂ snow may also be directly injected into the CO₂rich liquid.

The carbon dioxide-lean stream may instead be fed to a heat exchangeroptionally provided with a mechanical scraper. In such an embodiment,the carbon dioxide-lean stream is cooled to an exent that the gas phasecarbon dioxide is condensed into the solid phase. The mechanical scraperscrapes the solid carbon dioxide formed on the surface of the heatexchanger and feeds it to an auger screw which may serve to elevate thepressure of the solid carbon dioxide to its triple point (5.2 bar abs).The latent heat of liquefaction may be used to cool the flue gas or tocondense the carbon dioxide-rich stream after compression or a warmed,vaporized portion of the CO₂-rich liquid may be used to directly heatthe solid carbon dioxide to liquefy it. This solid condensation andsubsequent liquefaction may be performed in parallel with solidcondensation occurring at one heat exchanger while the solid carbondioxide in the other heat exchanger is simultaneously liquefied. Thethus-liquefied CO₂ may be mixed with the CO₂-rich liquid. Alternatively,the CO₂ snow may also be directly injected into the CO₂ rich liquid.

One of ordinary skill in the art will recognize that the CO₂ snow maycontain components other than CO₂. Other gaseous components may besolidified in the turbo expander or incorporated into the CO₂ snow asbubbles or drops. Therefore, the CO₂ snow may not be entirelyconstituted of CO₂.

Cold Absorption

In a third embodiment, the gas separation membrane produces a carbondioxide-lean stream containing approximately 2 mole % to approximately10 mole % carbon dioxide. As in the first and second embodiments, thecarbon dioxide-rich stream is compressed and cooled to produce a liquidvapor mixture that is subsequently separated to produce the CO₂ richliquid. The CO₂ concentration in the carbon dioxide-lean stream may bedecreased further by an absorption process which is also operated atsub-ambient temperature. Examples of a suitable sub-ambient absorptionprocesses are those based on methanol or chilled ammonia as described inU.S. Published Patent Application No. 2009/148930, WO06/022885, andWO09/073422, the absorption processes of which are hereby incorporatedherein by reference.

The carbon dioxide-lean stream may be sent directly to the chilledabsorption process before or after expansion. Preferably, the carbondioxide-lean stream is sent to the chilled absorption process beforeexpansion. The chilled absorption process is highly effective atreducing CO₂ to low percentage levels, typically ranging fromapproximately 0.5% v/v to approximately 2% v/v. The CO₂ absorptionprocess may be optimized through the pressure and temperature of thecarbon dioxide-lean stream. Typical pressures for the CO₂ absorptionstep with chilled ammonia may range from approximately 1 toapproximately 5 bar and temperatures may range from approximately 0° C.to approximately 15° C. Cold temperature operation also minimizesabsorption solvent (e.g., methanol, ammonia) loss in the vent gas. Dueto the lower CO₂ content in the carbon dioxide-lean stream, the energycosts for regenerating the absorption solvent are lower when compared tothe energy costs to regenerate the absorption solvent for theCO₂-containing gas mixture. The cold membrane process is more efficientat CO₂ recovery at relatively higher CO₂ concentrations. The hybrid coldmembrane and cold absorption configuration for CO₂ capture may be moreeconomical than either process alone.

Gas Turbine Embodiments

The fourth through sixth embodiments incorporate a modified gas turbinecomprising a compressor, combustion chamber, and turbine. Suitable gasturbines for modification are available from GE, Siemens, Mitsubishi, orAlstom. In these embodiments, the compressor of the gas turbine is usedto compress the CO₂-containing gas mixture. The gas turbine is modifiedso that the CO₂-containing gas mixture may be extracted from thecompressor after compression, rather than being directed to thecombustion chamber, and further modified so that the carbon dioxide-leanstream may be introduced to the combustion chamber. Expansion of thecarbon dioxide-lean stream or expansion of the products of combustionfrom the combustion chamber (combusted in the presence of the carbondioxide-lean stream) in the turbine provides mechanical energy to powerthe compressor.

The compressed CO₂-containing gas mixture is extracted from the gasturbine, cooled, and directed to the gas separation membrane module toproduce a carbon dioxide-rich stream and a carbon dioxide-lean stream.The carbon dioxide-lean stream is introduced to the combustion chamberof the gas turbine where it is mixed with air and a fuel of H₂ and/ornatural gas. The products of combustion of the air and fuel (in thepresence of the carbon dioxide-lean stream) are expanded in the turbine.Due to the relatively higher pressure of the carbon dioxide-lean stream,more mechanical energy is recovered. If the temperature of the expandedproducts of combustion/carbon dioxide-lean stream mixture exiting theturbine is sufficiently high, the mixture stream may be directed to aHeat Recovery Steam Generator (HRSG) to produce additional power. Duringnighttime when the price of electricity is generally lower than duringthe daytime, the consumption of H₂ or natural gas may be decreased andthe HRSG used as a motor to produce sufficient compression power to thegas turbine.

In the fourth embodiment, the carbon dioxide-lean stream may be warmedto a temperature ranging from approximately 100° C. to approximately200° C. so that it is at an ambient temperature at the outlet of theturbine, preventing the loss of heat to the atmosphere and itsaccompanying energy.

In the fifth embodiment, the carbon dioxide-lean stream may be heated toa temperature ranging from approximately 300° C. to approximately 750°C. Such heating may be accomplished by a boiler.

In the sixth embodiment, the carbon dioxide-lean stream may be heated toa temperature ranging from approximately 1000° C. to approximately 1400°C. To reach this temperature, natural gas or liquid fuel must be burned.

As in the first through third embodiments, the carbon dioxide-richstream is compressed (in a separate compressor) and cooled to produce aliquid vapor mixture that is subsequently separated to produce the CO₂rich liquid. In these embodiments, an external refrigerant may benecessary.

Defrosting

In all embodiments, whenever pressure drop or heat transfer limitationsbecome uneconomical and/or inefficient, a defrosting step mayoccasionally be utilized to remove any condensation and/orcrystallization products from the gas separation membrane and, whereapplicable, the heat exchanger. During the defrosting step, theCO₂-containing gas mixture flows through the gas separation membrane andthe heat exchanger at a temperature ranging from approximately 0° C. toapproximately 40° C. The “warm” CO₂-containing gas mixture is removedprior to reaching the cryogenic phase separation step.

EXAMPLES Example 1

FIG. 2A is an exemplary flow diagram of the first embodiment of thedisclosed method. A CO₂-containing gas mixture 1 is compressed bycompressor 20 to produce a compressed CO₂-containing gas mixture 5. Asindicated by the dotted line, the heat of compression may optionally becaptured in boiler feed water 25.

The compressed CO₂-containing gas mixture 5 may be subject to anynecessary treatment to render the mixture suitable for furtherprocessing. In FIG. 2, one such treatment is embodied as a drying step,in dryer 10. Any impurities, such as water, are removed from thecompressed CO₂-containing gas mixture 5 in the treatment step asimpurity stream 16 to the level required to prevent undesiredcondensation in heat exchanger 30.

The dried compressed CO₂-containing gas mixture 15 is then combined withstream 57 to provide stream 17 which is cooled in heat exchanger 30.Although this embodiment depicts only one heat exchanger 30, one ofordinary skill in the art will recognize that multiple heat exchangersmay replace the one heat exchanger 30 shown in FIG. 2. The cooled,dried, compressed CO₂-containing gas mixture 31 flows into gasseparation membrane 40 to produce a carbon dioxide-lean stream 42 and acarbon dioxide-rich stream 45 at a lower pressure.

Depending upon whichever other process streams are in heat exchangerelationship with the carbon dioxide-lean stream 42, the stream 42 maybe warmed by and provides cooling to heat exchanger 30 or is cooled byand provides warth to heat exchanger 30. The carbon dioxide-lean stream32 is then subjected to expansion by turboexpander 50. The resultingcold carbon dioxide-lean stream 55 is warmed by and provides cooling toheat exchanger 30. If, after passing through the heat exchanger 30, anyexcess pressure remains in the warmed expanded carbon dioxide-leanstream 33, the stream may be expanded at turboexpander 51 for mechanicalenergy recovery and the de-pressurized carbon dioxide-lean stream 56 maythen be vented.

The carbon dioxide-rich stream 45 is compressed by compressor 21. Thecompressed carbon dioxide-rich stream 47 is partially condensed in heatexchanger 30 to provide biphasic liquid/vapor carbon dioxide. Thebiphasic liquid/vapor carbon dioxide is subjected to phase separation invapor liquid separator 60. CO₂ rich liquid 75 is pumped by pump 70 fromthe separator 60 to the heat exchanger 30. The warmed CO₂ rich liquid 34is typically provided at greater than approximately 60 bar and atapproximately 20° C.

The separator 60 also yields CO₂ lean vapor stream 65. CO₂ lean vaporstream 65 is warmed in and provides cooling to heat exchanger 30. Thewarmed CO₂ lean vapor stream undergoes expansion in turboexpander 52.The warmed, expanded CO₂ lean vapor stream 57 is mixed with dried,compressed CO₂-containing gas mixture 15 prior to being cooled by heatexchanger 30. This recycle increases the CO₂ concentration of the feed31 to the membrane.

The process of FIG. 2A was simulated using chemical engineeringsimulation software HYSIS for air-fired combustion of coal. As seen inFIG. 2B, starting with a flue gas derived from air-fired combustion andutilizing the process of FIG. 2A, the system provides a liquid product34 having a carbon dioxide concentration of 97.2%.

FIG. 2C is an exemplary flow diagram of a variation of the firstembodiment of the disclosed method that was simulated using chemicalengineering simulation software HYSIS. Substantively, the scheme of FIG.2B differs from that of FIG. 2A in that the CO₂-lean vapor stream fromthe cryogenic phase separator is expanded before being cooled in thecommon heat exchanger and combined, with the dried compressedCO₂-containing gas mixture after cooling. Flue gas 1 is pressurizedusing axial compression to knock out water and provide a compressed fluegas 2. Multistage compression is required to reach the 16 bar feedpressure. The heat of the compression is removed by preheating boilerfeed water to yield a cooled compressed CO₂-containing gas mixture 3.After compression, the gas is cooled and dried using silica gel toprovide a dried feed gas 4. This gas 4 is cooled using a high efficiencyfinned multi-stream heat exchanger and fed to the membrane as cooledfeed gas 6. The CO₂ lean non-permeate (retentate) stream is thencryogenically turbo-expanded in a series of steps, and passed throughthe heat exchanger as a cold stream 14. The remaining pressure is thenexpanded in a heated turbo-expander (which recovers energy moreefficiently than a cryogenic turbo-expander) to yield vent stream 17.The CO₂ rich permeate is compressed to provide a compressed CO₂ richpermeate 7. After partial cooling, the now warm permeate 8 is re-cooledin the heat exchanger. This stream is cooled until partial condensationoccurs to provide partially condensed permeate 9 and separated in avapor liquid separator. The 95+% CO₂ liquid condensate 10S is thenpumped by a cryogenic pump to a pressure where it maintains a liquidstate at room temperature to provide stream 18, and then this stream 18is warmed in the heat exchanger to provide liquid carbon dioxide product19. The incondensible vapor 11 is expanded to the feed pressure of themembrane to provide stream 12, and warmed in the heat exchanger to thefeed temperature of the membrane. The vapor is recycled back to themembrane to provide recycled incondensible vapor 13 which operates as asweep gas to drive permeation of carbon dioxide from the cooled feed gas6 across the membrane.

Assuming the initial conditions for the flue gas 1, the HYSIS simulationyielded the temperature, pressure, flow, and carbon dioxideconcentration conditions in Table 2.

TABLE 2 Air-Fired Combustion of Coal Stream # 1 2 3 4 6 13 6 + 13Temperature [C.] 57.2 20.0 6.5 10.0 −30.0 −30.0 −30.0 Pressure [bar] 1.016.2 16.1 16.1 16.1 16.1 16.1 Molar Flow [Nm³/h] 1000 840 836 835 835 71906 CO₂ Concentration 14.1% 16.8% 16.9% 16.9% 16.9% 32.3% 18.1% Stream #Ret Perm 9 10 11 17 19 Temperature [C.] −30.0 −30.0 −55.0 −55.0 −55.015.0 8.0 Pressure [bar] 15.5 1.2 19.8 19.8 19.8 1.0 54.5 Molar Flow[Nm³/h] 705 202 202 131 71 705 131 CO₂ Concentration 1.9% 74.6% 74.6%97.4% 32.3% 1.9% 97.4%

As seen in Table 2, with little additional cooling, the system providesa liquid product 19 having a carbon dioxide concentration of 97.4%.

Example 2

A CO₂-containing gas mixture having 30% CO₂ with a balance of N₂ was fedto membranes comprising the Matrimid® and/or P84® polyimides. The dataare plotted in FIGS. 3 through 8 as normalized values with reference tothe performance at ambient temperature. The normalized CO₂ GPU equalsthe CO₂ GPU at cold temperature divided by the reference CO₂ GPU for thesame membrane at 22° C. and the normalized CO₂/N₂ selectivity equals theCO₂/N₂ selectivity at cold temperature divided by the reference CO₂/N₂selectivity for same membrane at 22° C. The reference CO₂/N₂ selectivityat 22° C. for the membrane comprising the Matrimid® polyimide is 36. Thereference CO₂ GPU at 22° C. for the membrane comprising the Matrimid®and P84® polyimides is 74 and its reference CO₂/N₂ selectivity is 39.

The results in FIGS. 3 and 4 for a membrane comprising the Matrimid®polyimide show increasing flux (approximately a 25% increase) andselectivity over the first 100 hrs at 200 psi, −40° C., and 196reference CO₂ GPU. After 1000 hrs, the permeance at −40° C. approachesthe room temperature permeance value with selectivity three times higherthan the room temperature selectivity value.

The results in FIGS. 5 and 6 for a membrane comprising the Matrimid®polyimide show increasing flux (approximately a 55% increase) andselectivity over the first 100 hours at 300 psi, −40° C., and 160reference CO₂ GPU. During the testing, feed gas pressure variationdeveloped and therefore the last two data points are not representative.Discounting these data points, the increase in flux is approximately150%.

The results in FIGS. 7 and 8 for a membrane comprising the Matrimid® andP84® polyimides show increasing flux (approximately a 25% increase) andselectivity over the first 100 hours at 200 psi and −40° C. After 1000hrs, the permeance at low temperature exceeds the room temperaturepermeance value with selectivity 3.5 times higher than the roomtemperature selectivity value.

In all of the testing, the CO₂ permeance rises with time, implyingpolymer/gas interactions. However, the selectivity remains roughlyconstant or increases, which discounts classical plasticization as amechanism.

Example 3

FIG. 9A depicts an exemplary flow diagram of a variation of the firstembodiment of the disclosed method. In this example, after being warmedby and providing cooling to heat exchanger 30, a portion of the coldcarbon dioxide-lean stream 55 is utilized as a sweep stream 37 in thegas separation membrane 40 to lower the partial pressure of CO₂permeating through the membrane 40 from the cooled and compressedCO₂-containing gas mixture 31.

The process of FIG. 9A was simulated using chemical engineeringsimulation software HYSIS for four different schemes: a) air-firedcombustion of coal, b) partial oxycombustion of coal with high airinfiltration, c) partial oxycombustion of coal with low airinfiltration, and d) full oxycombustion of coal.

As seen in FIG. 9B, starting with a flue gas derived from air-firedcombustion and utilizing the process of FIG. 9A, the system provides aliquid product 34 having a carbon dioxide concentration of 97.4%.

As seen in Table 3, starting with a flue gas derived from partialoxycombustion of coal with high air infiltration and utilizing theprocess of FIG. 9A, the system provides a liquid product 34 having acarbon dioxide concentration of 98.07%.

TABLE 3 Partial Oxycoal Combustion With High Air Infiltration Stream # 131 37 42 45 36 34 65 Vapor Phase 1 1 1 1 1 0.26 0 1 Fraction Temp (° C.)58 −40 −40 −59.2 −59.2 −54 20 −54 Pressure (bar) 1.1 15.9 2.9 15.3 2.916.3 150.0 16.3 Molar Flow 1000 935 40 559 416 416 307 109 (Nm³/h(gas))Mass Flow 1424 1453 50 736 768 768 599 169 (kg/h) Molar fractions %N₂(v/v) 44.40 53.32 95.0 85.4 14.19 14.19 1.45 50.03 % O2 (v/v) 3.404.74 5.00 6.21 2.79 2.79 0.41 9.48 % Argon (v/v) 1.60 1.85 0.00 2.820.37 0.37 0.08 1.19 % CO₂ (v/v) 33.20 40.09 0.00 5.56 82.66 82.66 98.0739.30 % H₂O (v/v) 17.40 0.00 0.00 0.00 0.00 0.00 0.00 0.00

As seen in Table 4, starting with a flue gas derived from partialoxycombustion of coal with low air infiltration and utilizing theprocess of FIG. 9A, the system provides a liquid product 34 having acarbon dioxide concentration of 98.76%.

TABLE 4 Partial Oxycoal Combustion With High Air Infiltration Stream # 131 37 42 45 36 34 65 Vapor Phase 1 1 1 1 1 0.23 0 1 Fraction Temp (° C.)58 −40 −40 −49.2 −49.2 −54 20 −54 Pressure (bar) 1.1 11.4 3.0 10.8 3.012.3 150.0 12.3 Molar Flow 1000 952 40 436 557 557 431 125 (Nm³/h(gas))Mass Flow 1525 1590 50 593 1048 1048 843 205 (kg/h) % N₂(v/v) 29.9436.62 95.00 76.63 9.51 9.51 0.86 39.25 % O₂ (v/v) 3.76 5.21 5.00 8.782.39 2.39 0.31 9.54 % Argon (v/v) 2.00 2.28 0.00 4.52 0.37 0.37 0.071.39 % CO₂ (v/v) 46.96 55.89 0.00 10.07 87.73 87.73 98.76 49.82 % H₂O(v/v) 17.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00

As seen in FIG. 9E, starting with a flue gas derived from air fulloxycombustion of coal and utilizing the process of FIG. 9A, the systemprovides a liquid product 34 having a carbon dioxide concentration of98.68%.

TABLE 5 Fully Oxycoal Combustion Stream # 1 31 37 42 45 36 34 65 VaporPhase 1 1 1 1 1 0.14 200 1 Fraction Temperature 57 −40 −40 −47.5 −47.5−54 20 −54 (° C.) Pressure (bar) 1.05 8.9 3.4 8.4 3.42 12.3 150.0 12.3Molar Flow 1000 1034 30 340 724 724 621 103 (Nm³/h(gas)) Mass Flow 17251839 38 493 1384 1384 1215 169 (kg/h) % N₂(v/v) 18.06 20.80 95.0 60.175.38 5.38 0.73 33.44 % O₂ (v/v) 4.59 5.87 5.00 13.10 2.43 2.43 0.4714.31 % Argon (v/v) 2.69 2.85 0.00 7.70 0.46 0.46 0.12 2.49 % CO₂ (v/v)67.76 70.48 0.00 19.03 91.73 91.73 98.68 49.76 % H₂O (v/v) 6.89 0.000.00 0.00 0.00 0.00 0.00 0.00

Thus, regardless of whether the process of FIG. 9A is retrofitted forexisting air-fired coal power plants, retrofitted with partialoxycombustion on existing air-fired coal power plant, implemented in apartial oxycombustion coal power plant, or implemented in a fulloxycombustion coal power plant, the process of FIG. 9A achieves in eachcase a CO₂ purity of at least 97%.

Example 4

Under certain feed conditions, computer models indicate that thedisclosed method is more energy efficient and requires less membranearea when employing a sweep stream having a low CO₂ concentration. Inthe computer model, a comparison of the method with (the sweep case) andwithout (the base case) a sweep stream was set to provide 90% recoveryof CO₂. The sweep stream utilized was 3.5% of the expanded carbondioxide-lean stream. The fiber performance was set to 90 GPU CO₂/1 GPUN₂/2 GPU Ar/6 GPU O₂. The energy (in KWhr/ton) is defined to be theamount of energy required to process 1 ton of CO₂ capture. The base caserequires an net energy consumption of 200 kWhr/ton CO2 compared to 196kWhr/ton CO₂ for the sweep case. The sweep case also requires 29% lessmembrane area than the base case.

Example 5

FIG. 10A depicts an exemplary flow diagram of the second embodiment ofthe disclosed method. In this embodiment, the warmed carbon dioxide-leanstream 32, the turboexpander 50, the cold carbon dioxide-lean stream 55,the warmed expanded carbon dioxide-lean stream 33, the turboexpander 51,and the de-pressurized carbon dioxide-lean stream 56 of FIG. 2 have beenremoved. In this embodiment, the carbon dioxide-lean stream 42 isexpanded in a cryogenic turboexpander 53 to produce a biphasicsolid/gaseous carbon dioxide-lean stream 58.

The biphasic solid/gaseous carbon dioxide-lean stream 58 exits theturboexpander 53 and is directed into gas/solid separator 80, yieldingCO₂ snow 81 and CO₂ depleted gas 82. As indicated by the dotted lines,the CO₂ depleted gas 82 may be vented, may be utilized to provideadditional cooling to the heat exchanger 30, or a combination of both.The CO₂ snow is mixed with CO₂ rich liquid 75 prior to pump 70.

As with the first embodiment, after a reduction in its pressure, aportion of the carbon dioxide-lean stream 58 may be utilized as a sweepstream (not shown) in the gas separation membrane 40 to lower thepartial pressure of CO₂ permeating through the membrane 40 from thecooled and compressed CO₂-containing gas mixture 31. Similarly, the CO₂depleted gas 82 may used as a sweep stream (not shown).

FIG. 10B depicts an exemplary flow diagram of a variation of the secondembodiment of the disclosed method. A CO₂-containing gas mixture 101 iscompressed by compressor 190 to produce a compressed CO₂-containing gasmixture 102. The heat of compression may optionally be captured inboiler feed water.

The compressed CO₂-containing gas mixture 102 may be subject to anynecessary treatment to render the mixture suitable for furtherprocessing. Any impurities, such as water, are removed from thecompressed CO₂-containing gas mixture 102 to the level required toprevent undesired condensation in heat exchanger 30.

The dried compressed CO₂-containing gas mixture 102 is then combinedwith stream 107 and cooled in heat exchanger 30. Although thisembodiment depicts only one heat exchanger 30, one of ordinary skill inthe art will recognize that multiple heat exchangers may replace the oneheat exchanger 30 shown in FIG. 10B. The cooled, dried, compressedCO₂-containing gas mixture 103 flows into gas separation membrane M toproduce a carbon dioxide-lean stream 104 and a carbon dioxide-richstream 105 at a lower pressure.

The carbon dioxide-lean stream 104 is cooled by heat exchanger 30. Thecooled carbon dioxide-lean stream 109 is then subjected to expansion bycryogenic turboexpander 153 to produce solid carbon dioxide and aCO₂-depleted gas in phase separator 180.

The CO₂-depleted gaseous stream 110 from phase separator 180 is thenwarmed at heat exchanger 30 and expanded at cold expander 126. Theexpanded CO₂-depleted gaseous stream 110 is again warmed by the heatexchanger 30 to provide lower-pressure CO-depleted stream 112. Thisstream 112 is heated with steam 130 and expanded at hot expander 120 toambient or near-ambient pressure. The lower pressure, expanded stream114 may be vented or a portion or all of the lower pressure, expandedstream 114 may be directed to the permeate side of the membrane M whereit is utilized as a sweep stream in the gas separation membrane M tolower the partial pressure of CO₂ permeating through the membrane M fromthe cooled and compressed CO₂-containing gas mixture 103.

The carbon dioxide-rich stream 105 is compressed by compressor 191. Thecompressed carbon dioxide-rich stream 105 is partially condensed bycooling in heat exchanger 30. The partially condensed carbondioxide-rich stream 106 is directed to liquid vapor separator S. Aportion of the CO₂ rich liquid 108 is pumped by pump 160 to provideliquid carbon dioxide product 115. Two portions 108A, 108B of the CO₂rich liquid 108 are pressure-reduced at pressure reduction valves 109A,109B and vaporized by heat exchanger 30. The first portion of vaporizedcarbon dioxide rich liquid is directed to phase separator 180 where itliquefies the solid carbon dioxide to provide liquid carbon dioxidestream 111. The second portion of vaporized carbon dioxide rich liquidis expanded at expander 140 and condensed with cooling water 150 toprovide an additional stream of liquid carbon dioxide. This additionalstream of liquid carbon dioxide is pumped by pump 160 and combined withliquid carbon dioxide stream 111 to provide another source of the liquidcarbon dioxide product 115.

The process of FIG. 10B was simulated using chemical engineeringsimulation software HYSIS to yield the following vapor fraction,temperature, pressure, flow, and mole fraction values recited in Table6.

TABLE 6 Solid Condensation and Liquefaction Stream # 101 102 103 104 105Vapor Fraction %  100%  100%  100%  100%  100% Temperature C.    50.00   6.49   −30.00   −30.69   −30.00 Pressure bar a    1.05    12.00   11.90    11.39    1.50 Molar Flow Nm3/h 1,000   834 904 756 177 MassFlow kg/h 1,279   1,146   1,246   975 309 Mole (N₂) % 66.95%  80.23% 79.33%  91.77%  28.84%  Mole (O₂) % 2.32% 2.78% 3.13% 3.12% 3.19% Mole(Ar) % 0.80% 0.96% 0.97% 1.08% 0.50% Mole (CO₂) % 13.30%  15.93% 16.57%  4.03% 67.48%  Mole (H₂O) % 16.63%  0.10% 0.00% 0.00% 0.00%Stream # 106 107 108 109 110 Vapor Fraction %  39%  100%   0%  100% 100% Temperature C.   −55.00   −55.00   −55.00   −87.78   −104.91Pressure bar a    29.85    29.85    29.85    11.29    5.00 Molar FlowNm3/h 177  70 108 756 737 Mass Flow kg/h 309 101 208 975 938 Mole (N₂) %28.84%  67.70%  3.61% 91.77%  94.06%  Mole (O₂) % 3.19% 7.24% 0.57%3.12% 3.20% Mole (Ar) % 0.50% 1.07% 0.12% 1.08% 1.11% Mole (CO₂) %67.48%  24.00%  95.70%  4.03% 1.63% Mole (H₂O) % 0.00% 0.00% 0.00% 0.00%0.00% Stream # 111 112 113 114 115 Vapor Fraction %   0%  100%  100% 100%   0% Temperature C.   −55.86    11.02   −34.03    48.32    16.76Pressure bar a    5.20    4.02    1.53    1.01    75.00 Molar Flow Nm3/h 18 708  29 708 126 Mass Flow kg/h  36 901  38 901 244 Mole (N₂) % 0.00%94.06%  94.06%  94.06%  3.08% Mole (O₂) % 0.00% 3.20% 3.20% 3.20% 0.48%Mole (Ar) % 0.00% 1.11% 1.11% 1.11% 0.11% Mole (CO₂) % 100.00%  1.63%1.63% 1.63% 96.32%  Mole (H₂O) % 0.00% 0.00% 0.00% 0.00% 0.00%

As seen in Table 6, starting with a flue gas having the aboveproperties, the process of FIG. 10B provides a liquid product 34 havinga carbon dioxide concentration of 96.32% with little additional cooling.

Example 6

FIG. 11 depicts an exemplary flow diagram of the third embodiment of thedisclosed method. In this embodiment, the warmed carbon dioxide-leanstream 32, the turboexpander 50, the cold carbon dioxide-lean stream 55,the warmed expanded carbon dioxide-lean stream 33, the turboexpander 51,and the de-pressurized carbon dioxide-lean stream 56 of FIG. 2 have beenremoved. In this embodiment, the carbon dioxide-lean stream 42 is sweptover an absorption vessel 90. One of ordinary skill in the art willrecognize, that although only one absorption vessel 90 is depicted inFIG. 11, that multiple absorption vessels may be utilized.

During the adsorption step, the adsorption vessel 90 produces a CO₂ leanabsorption stream 91. During the desorption step, the adsorption vessel90 produces a CO₂ rich stream 92. The treated CO₂ lean stream 91 may besent to vent. The desorbed CO₂ rich stream 92 is compressed incompressor 22 to produce a product CO₂ stream, which may be sequesteredor used elsewhere.

Example 7

FIG. 12A depicts an exemplary flow diagram of the fourth through sixthembodiments of the disclosed method. In these embodiments, thecompressor 20, the turboexpander 50, the cold carbon dioxide-lean stream55, the warmed expanded carbon dioxide-lean stream 33, the turboexpander51, and the de-pressurized carbon dioxide-lean stream 56 of FIG. 2 havebeen removed. In these embodiments, the compressor 20 has been replacedby modified gas turbine 95.

The CO₂-containing gas mixture 1 is compressed by the modified gasturbine 95 to produce a compressed CO₂-containing gas mixture 5. Anyimpurities, such as water, are removed from the compressedCO₂-containing gas mixture 5 in a treatment step, such as dryer 10, asimpurity stream 16 to the level required to prevent undesiredcondensation in heat exchanger 30.

The dried compressed CO₂-containing gas mixture 15 is cooled in heatexchanger 30. Although this embodiment schematically depicts only oneheat exchanger 30, one of ordinary skill in the art will recognize thatmultiple heat exchangers may replace the single heat exchanger 30schematically shown in FIG. 12A. The cooled, dried, compressedCO₂-containing gas mixture 31 flows into gas separation membrane 40 toproduce a carbon dioxide-lean stream 42 and a carbon dioxide-rich stream45 at a lower pressure. Processing of the carbon dioxide-rich stream 45occurs as described with respect to FIG. 2, except that a cooling source98 may be required to provide sufficient cooling to the heat exchanger30. Suitable cooling sources 98 may include liquid nitrogen, part of thewarmed CO₂ rich liquid 34, or other cooling sources known in the art.

The carbon dioxide-lean stream 42 is warmed by and provides cooling toheat exchanger 30. The warmed carbon dioxide-lean stream 32 returns tothe modified gas turbine 95 where it may be combined with a secondcomponent 96. The second component includes air and a fuel of H₂ and/ornatural gas. The combined warmed carbon dioxide-lean stream 32/secondcomponent 96 are directed to a combustion chamber associated with themodified gas turbine 95 whereat the fuel is combusted with the air inthe presence of the warmed carbon dioxide-lean stream. The products ofcombustion, at enhanced pressure due to the presence of the relativelyhigher pressure carbon dioxide-lean stream, are expanded at the modifiedgas turbine 95 to produce mechanical energy for compression of theCO₂-containing gas mixture 1. The expanded gas 97 may be vented with orwithout first being used to preheat the air and fuel. The expanded gas97 can also be cooled in the heat exchanger 30 and utilized as a sweepgas on the permeate side of the gas separation membrane 40.

FIG. 12B depicts an exemplary flow diagram of a variation of the fourththrough sixth embodiments of the disclosed method.

The CO₂-containing gas mixture 101 is compressed by compressor 190 toproduce a compressed CO₂-containing gas mixture 102. Any impurities,such as water, are removed from the compressed CO₂-containing gasmixture in a treatment step, such as dryer, to the level required toprevent undesired condensation in heat exchanger 30.

The dried compressed CO₂-containing gas mixture is cooled in heatexchanger 30. Although this embodiment schematically depicts only oneheat exchanger 30, one of ordinary skill in the art will recognize thatmultiple heat exchangers may replace the single heat exchanger 30schematically shown in FIG. 12B. The cooled, dried, compressedCO₂-containing gas mixture 103 flows into gas separation membrane M toproduce a carbon dioxide-lean non-permeate 104 and a carbon dioxide-richpermeate 105 at a lower pressure.

The carbon dioxide-lean stream 104 is warmed by and provides cooling toheat exchanger 30. The warmed carbon dioxide-lean stream 104 is furtherwarmed at heat exchanger 175. Downstream of heat exchanger 175, thetwice-warmed carbon dioxide-lean stream 104, a fuel 171, and air 172(which are compressed at compressors 173, 174, respectively) areintroduced to a combustion chamber of a gas turbine 176 whereat the fuelis combusted with the oxidant in the presence of the warmed carbondioxide-lean stream. Although the fuel 171, air 172, and twice-warmedcarbon dioxide-lean stream 104 are typically separately introduced tothe combustion chamber, for sake of performing a mass balance, theircombination may be considered to be stream 178. The products ofcombustion, at enhanced pressure due to the presence of the relativelyhigher pressure carbon dioxide-lean stream, are expanded at the gasturbine 176 to produce mechanical energy for compression of theCO₂-containing gas mixture 101 or of the carbon dioxide-rich permeate105. The expanded products of combustion 177 may be vented to atmosphereas stream 182. A portion of the expanded products of combustion 177 arecooled in the heat exchanger 30 to provide sweep gas 113 on the permeateside of the gas separation membrane M.

The carbon dioxide-rich permeate 105 is compressed at compressor 191 andcooled at heat exchanger 30. The degree of cooling at heat exchange issufficient to partially condense CO₂ in the carbon dioxide-rich permeate105. The liquid and vapor phases are then separated at separator S. TheCO₂-lean vapor stream 107 provides cold energy for cooling theCO₂-containing gas mixture and partially condensing the compressedcarbon dioxide-rich permeate 105. After being warmed in the heatexchanger, the CO₂-lean vapor stream 107 is then combined with theCO₂-containing gas mixture 101 downstream of compressor 190 for coolingat heat exchanger 30 and introduction to gas separation membrane M.

Additional cold energy for cooling the CO₂-containing gas mixture andpartially condensing the compressed carbon dioxide-rich permeate 105 isprovided by two portions of the carbon dioxide-rich liquid 108 that aresubjected to pressure reduction at pressure reduction valves 109A, 109Bto provide two portions of pressure-reduced carbon dioxide-rich liquid108A, 108B. After being vaporized in the heat exchanger, the now-gaseousportions of carbon dioxide-rich fluid are compressed at compressors140A, 140B, and condensed at heat exchangers 105A, 150B (cooled bycooling water) in order to provide carbon dioxide liquid product CDLP.The remainder of the carbon dioxide-rich liquid 108 is pumped at pump160 to provide the remainder of the carbon dioxide liquid product CDLP.

The process of FIG. 12B was simulated using chemical engineeringsimulation software HYSIS to yield the following vapor fraction,temperature, pressure, flow, and mole fraction values recited in Table7.

TABLE 7 Parameters for streams in process of FIG. 12B Stream # 101 102103 104 105 106 Vapour Fraction %  100%  100%  100%  100%  100%  56%Temperature C.    50.00    6.49   −30.00   −32.15   −30.00   −55.00Pressure bar a    1.05    16.10    16.00    15.82    1.50    20.85 MolarFlow Nm³/h 1,000   834 1,004   737 305 305 Mass Flow kg/h 1,279  1,146   1,400   930 517 517 Mole Frac (N₂) % 66.95%  80.25%  76.73% 95.04%  34.10%  34.10%  Mole Frac (O₂) % 2.32% 2.78% 3.77% 3.06% 5.02%5.02% Mole Frac (Ar) % 0.80% 0.96% 0.99% 1.11% 0.66% 0.66% Mole Frac(CO₂) % 13.30%  15.93%  18.51%  0.45% 60.21%  60.21%  Mole Frac (H₂O) %16.63%  0.08% 0.00% 0.34% 0.00% 0.00% Mole Frac (CH₄) % 0.00% 0.00%0.00% 0.00% 0.00% 0.00% Stream # 107 108 172 171 178 177 Vapour Fraction%  100%   0%  100%  100%  100%  100% Temperature C.   −55.00   −55.00   20.00    20.00   1392.84   685.77 Pressure bar a    20.85    20.85   1.00    1.00    15.65    1.06 Molar Flow Nm³/h 171 135 180  30 947947 Mass Flow kg/h 255 262 232  21 1,184   1,184   Mole Frac (N₂) %59.25%  2.19% 79.00%  0.00% 88.98%  88.98%  Mole Frac (O₂) % 8.61% 0.47%21.00%  0.00% 0.04% 0.04% Mole Frac (Ar) % 1.11% 0.09% 0.00% 0.00% 0.87%0.87% Mole Frac (CO₂) % 31.03%  97.24%  0.00% 0.00% 3.52% 3.52% MoleFrac (H₂O) % 0.00% 0.00% 0.00% 0.00% 6.60% 6.60% Mole Frac (CH₄) % 0.00%0.00% 0.00% 100.00%  0.00% 0.00% Stream # 182 113 109A 109B CDLP VapourFraction %  96%  93%   7%   3%   0% Temperature C.    20.00   −34.03  −62.33   −56.84    20.00 Pressure bar a    1.01    1.52    6.00   11.00    75.00 Molar Flow Nm³/h 909  38  67  67 135 Mass Flow kg/h1,136    47 131 131 262 Mole Frac (N₂) % 88.98%  88.98%  2.19% 2.19%2.19% Mole Frac (O₂) % 0.04% 0.04% 0.47% 0.47% 0.47% Mole Frac (Ar) %0.87% 0.87% 0.09% 0.09% 0.09% Mole Frac (CO₂) % 3.52% 3.52% 97.24% 97.24%  97.24%  Mole Frac (H₂O) % 6.60% 6.60% 0.00% 0.00% 0.00% MoleFrac (CH₄) % 0.00% 0.00% 0.00% 0.00% 0.00%

As seen in Table 7, starting with a flue gas having the aboveproperties, the process of FIG. 12B provides a liquid product CDLPhaving a carbon dioxide concentration of 97.24% with little additionalcooling.

It will be understood that many additional changes in the details,materials, steps, and arrangement of parts, which have been hereindescribed and illustrated in order to explain the nature of theinvention, may be made by those skilled in the art within the principleand scope of the invention as expressed in the appended claims. Thus,the present invention is not intended to be limited to the specificembodiments in the examples given above and/or the attached drawings.

What is claimed is:
 1. A method of obtaining carbon dioxide from aCO₂-containing gas mixture, said method comprising the steps of:obtaining a CO₂-containing gas mixture; cooling the gas mixture to atemperature of from −20° C. to −50° C.; flowing the cooled gas mixtureinto a gas separation membrane module; directing a sweep gas to apermeate side of the membrane, the sweep gas having a low CO₂concentration; recovering a carbon dioxide-rich permeate from themembrane; compressing the carbon dioxide-rich stream; at least partiallycondensing the compressed carbon dioxide-rich stream by cooling toproduce a CO₂ rich liquid and a CO₂ lean vapor stream; and recovering acarbon dioxide-lean non-permeate from the membrane.
 2. The method ofclaim 1, further comprising the step of compressing the CO₂-containinggas mixture to a pressure ranging from about 3 bar to about 60 bar priorto the cooling step.
 3. A method of obtaining carbon dioxide from aCO₂-containing gas mixture, said method comprising the steps of:obtaining a CO₂-containing gas mixture; cooling the gas mixture; flowingthe cooled gas mixture into a gas separation membrane module; directinga sweep gas to a permeate side of the membrane, the sweep gas having alow CO₂ concentration; recovering a carbon dioxide-rich permeate fromthe membrane; recovering a carbon dioxide-lean non-permeate from themembrane; compressing the carbon dioxide-rich stream; at least partiallycondensing the compressed carbon dioxide-rich stream by cooling toproduce a CO₂ rich liquid and a CO₂ lean vapor stream; expanding the CO₂lean vapor stream; and warming the expanded CO₂ lean vapor stream, thesweep gas being the warmed expanded CO₂ lean vapor stream.
 4. A methodof obtaining carbon dioxide from a CO₂-containing gas mixture, saidmethod comprising the steps of: obtaining a CO₂-containing gas mixture;cooling the gas mixture; flowing the cooled gas mixture into a gasseparation membrane module; directing a sweep gas to a permeate side ofthe membrane, the sweep gas having a low CO₂ concentration; recovering acarbon dioxide-rich permeate from the membrane; recovering a carbondioxide-lean non-permeate from the membrane; cooling the carbondioxide-lean non-permeate; expanding the cooled carbon dioxide-leannon-permeate; and warming the expanded cooled carbon dioxide-leannon-permeate, the sweep gas being a portion of the warmed expandedcooled carbon dioxide-lean non-permeate.
 5. The method of claim 3,wherein at least 90% of the CO₂ in the CO₂-containing gas mixture isrecovered in the CO₂ rich liquid.
 6. The method of claim 1, furthercomprising the step of expanding the carbon dioxide lean stream to yielda pressure-reduced carbon dioxide lean stream having a temperature offrom −30° C. to −60° C.
 7. The method of claim 6, further comprising thestep of warming the expanded carbon dioxide lean stream at a heatexchanger, the warmed expanded carbon dioxide lean stream being thesweep gas.
 8. The method of claim 1, wherein the CO₂-containing gasmixture is obtained from the flue gas of a combustion process, from anatural gas stream, or from a CO₂ exhaust of an fermentative ethanolproduction plant.
 9. The method of claim 8, the CO₂-containing gasmixture is obtained from the flue gas of a combustion process and thecombustion process is selected from the group consisting of a steammethane reforming (SMR) process, a blast furnace, and air-fired oroxygen-enhanced fossil fuel combustion processes.
 10. The method ofclaim 9, wherein the combustion process is an oxygen-enhanced fossilfuel combustion process operated in full oxycombustion or partialoxycombustion mode.
 11. The method of claim 10, wherein theoxygen-enhanced fossil fuel combustion process is operated in fulloxycombustion mode, primary and secondary oxidants thereof being pureoxygen or synthetic air comprising oxygen and recycled flue gas.
 12. Themethod of claim 10, wherein the oxygen-enhanced fossil fuel combustionprocess is operated in partial oxycombustion mode, a primary oxidantthereof being air and a secondary oxidant thereof being synthetic aircomprising oxygen and recycled flue gas.
 13. The method of claim 9,wherein the combustion process is an air-fired fossil fuel combustionprocess, the fossil fuel is coal, and the CO₂-containing gas mixturecomprising about 8% v/v to about 16% v/v CO₂.
 14. The method of claim 9,wherein the combustion process is an air-fired fossil fuel combustionprocess, the fossil fuel is natural gas, and the CO₂-containing gasmixture comprising about 3% v/v to about 10% v/v CO₂.
 15. The method ofclaim 11, wherein the CO₂-containing gas mixture comprising about 60%v/v to about 90% v/v CO₂.
 16. The method of claim 9, wherein thecombustion process is a steam methane reforming (SMR) process, and theCO₂-containing gas mixture comprises about 15% v/v to about 90% v/v CO₂.17. The method of claim 9, wherein the combustion process is a blastfurnace, and the CO₂-containing gas mixture comprises about 20% v/v toabout 90% v/v CO₂.
 18. A method of obtaining carbon dioxide from aCO₂-containing as mixture, said method comprising the steps of:obtaining a CO₂-containing as mixture; cooling the gas mixture to atemperature of from 5° C. to −120° C.; flowing the cooled gas mixtureinto a gas separation membrane module; directing a sweep gas to apermeate side of the membrane, the sweep gas having a low CO₂concentration; recovering a carbon dioxide-rich permeate from themembrane; compressing the carbon dioxide-rich stream; at least partiallycondensing the compressed carbon dioxide-rich stream by cooling toproduce a CO₂ rich liquid and a CO₂ lean vapor stream; recovering acarbon dioxide-lean non-permeate from the membrane; introducing thecarbon dioxide lean stream to a combustion chamber of a gas turbinewhereat a fuel and an oxidant are combusted; directing the products ofcombustion from an outlet of the gas turbine to a heat exchanger; andexchanging heat between the colder fuel, oxidant, and carbon dioxidelean stream upstream of the gas turbine with the warmer products ofcombustion at the heat exchanger.
 19. The method of claim 18, furthercomprising the step of venting a portion of the cooled products ofcombustion to the atmosphere.
 20. The method of claim 18, furthercomprising the steps of: compressing the carbon dioxide-rich stream; atleast partially condensing the compressed carbon dioxide-rich permeateby cooling to produce a CO₂ rich liquid and a CO₂ lean vapor stream; andcooling a portion of the cooled products of combustion with cold energyfrom the CO₂-rich liquid, the cooled portion of cooled products ofcombustion being the sweep gas.
 21. The method of claim 1, wherein thesweep gas has a CO₂ concentration lower than that of the gas mixture.22. A method of obtaining carbon dioxide from a CO₂-containing asmixture, said method comprising the steps of: obtaining a CO₂-containinggas mixture; cooling the gas mixture to a temperature of from 5° C. to−120° C.; flowing the cooled gas mixture into a gas separation membranemodule; directing a sweep gas to a permeate side of the membrane, thesweep gas having a low CO₂ concentration; recovering a carbondioxide-rich permeate from the membrane; compressing the carbondioxide-rich stream; at least partially condensing the compressed carbondioxide-rich permeate by cooling to produce a CO₂ rich liquid and a CO₂lean vapor stream; recovering a carbon dioxide-lean non-permeate fromthe membrane; cooling the carbon dioxide-lean non-permeate; expandingthe cooled carbon dioxide-lean non-permeate at a cryogenic expander toproduce solid carbon dioxide and a CO₂-depleted gas in a phaseseparator; warming the CO₂-depleted gas; expanding the warmedCO₂-depleted gas at a cold expander; warming the expanded warmedCO₂-depleted gas; heating the warmed expanded warmed CO₂-depleted gasvia heat exchange with steam; and expanding the heated warmed expandedwarmed CO₂-depleted gas at hot expander to ambient or near-ambientpressure, a portion of which is used as the sweep gas.